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| general description the max8728 generates all the supply rails for thin-film transistor (tft) liquid-crystal display (lcd) panels in tvs and monitors. it includes step-down and step-up regula- tors, positive and negative charge pumps, and a dual- mode, logic-controlled high-voltage switch control block. the max8728 can operate from input voltages from 7v to 13.2v and is optimized for lcd tv panel and lcd moni- tor applications running directly from 12v supplies. the step-up and step-down regulators feature internal power mosfets and high-frequency operation allow- ing the use of small inductors and capacitors, resulting in a compact solution. both switching regulators use fixed-frequency, current-mode control architectures, providing fast load-transient response and easy com- pensation. the positive and negative charge-pump reg- ulators provide tft gate-driver supply voltages. both output voltages can be adjusted with external resistive voltage-dividers. the max8728 is available in a small (5mm x 5mm), low- profile (0.8mm), 32-pin tqfn package and operates over the -40? to +85? temperature range. applications lcd monitors lcd tvs features ? optimized for 10.8v to 13.2v input supply ? 7v to 13.2v input supply range ? selectable frequency (500khz/1mhz/1.5mhz) ? current-mode step-down regulator 14v internal n-channel mosfet 1.5% accurate output ? current-mode step-up regulator 19v internal n-channel mosfet 1% accurate output true shutdown (output goes to zero) ? 180 out-of-phase switching ? adjustable positive/negative charge pumps ? soft-start and timer delay fault latch for all outputs ? logic-controlled, high-voltage switches ? power-up and power-down sequences ? thermal-overload protection max8728 low-cost, multiple-output power supply for lcd monitors/tvs ________________________________________________________________ maxim integrated products 1 ordering information max8728 av dd v gon gate inl v in out1 v goff v l v l lx1 on/off out1 fbi fsel gnd1 gnd fbp src ref v cc gon drn mode thr comp shdn lcd enable en from t con ctl del fb2 lx2 gnd2 fbn drvn supp drvp vl bst in v in simplified operating circuit 19-3910; rev 0; 1/06 for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. part temp range pin- package package code MAX8728ETJ+ -40? to +85? 32 tqfn-ep* 5mm x 5mm t3255-4 pin configuration max8728 tqfn top view 29 30 28 27 12 11 13 out1 ctl in lx1 bst 14 gnd1 fb2 drn gon fbn src fsel 12 en 4567 23 24 22 20 19 18 mode del fbp ref gnd shdn drvn thr 3 21 31 10 comp v cc 32 9 fb1 vl gate 26 15 gndp lx2 25 16 drvp inl supp 8 17 gnd2 + denotes lead-free package. * ep = exposed pad. true shutdown is a trademark of maxim integrated products, inc.
max8728 low-cost, multiple-output power supply for lcd monitors/tvs 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (circuit of figure 1, v in = v inl = v supp = 12v, v out1 = +3.3v, v src = 28v, gnd1 = gnd2 = gndp = gnd = 0, i ref = 0, t a = 0? to +85? . typical values are at t a = +25?, unless otherwise noted.) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. in, inl, supp to gnd ............................................-0.3v to +14v supp to in ..........................................................................?.3v drvp to gndp.........................................-0.3v to v supp + 0.3v ctl, en, shdn , out1, v l , v cc to gnd ................-0.3v to +6v comp, fb1, fb2, fbn, fbp, fsel, del, thr, mode, ref to gnd ........................-0.3v to v cc + 0.3v gnd1, gnd2, gndp to gnd.............................................?.3v bst to gnd1 ..........................................................-0.3v to +20v lx1 to bst................................................................-6v to +0.3v lx2 to gnd2 ..........................................................-0.3v to +19v drvn, lx1, gate to gnd1 ..........................-0.3v to v in + 0.3v gon, src to gnd .................................................-0.3v to +40v src to gon ...........................................................-0.3v to +40v src to supp ..........................................................-0.3v to +30v src to supp (momentary)......................................-14v to +30v gon to supp ..........................................................-14v to +30v src to drn............................................................-0.3v to +40v drn to gnd ...........................................................-0.3v to +40v gon to drn...........................................................-0.3v to +30v v l short circuit to gnd..............................................momentary ref short circuit to gnd ...........................................continuous drvn rms current..........................................................-400ma drvp rms current.........................................................+100ma lx2 rms current ................................................................+1.6a gnd2 rms current ............................................................+1.6a lx1 rms current .................................................................-1.6a continuous power dissipation (t a = +70?) 32-pin thin qfn (derate 34.5mw/? above +70?) .....2758mw operating temperature range ...........................-40? to +85? junction temperature ......................................................+150? storage temperature range .............................-65? to +160? lead temperature (soldering, 10s) .................................+300? parameter conditions min typ max units general in, inl input voltage range for vl regulator operation 7.0 12.0 13.2 v inl quiescent current v fb2 = v fbp = 2.2v, v fbn = 0, lx2 not switching, lx1 switching 7ma in standby supply current v in = 7v to 13.2v, en = shdn = gnd 0.5 ma fsel = gnd 1275 1500 1730 fsel = v cc 850 1000 1150 switching frequency fsel = ref 425 530 610 khz phase difference between step-down/positive and step-up/negative regulators 180 degrees vl regulator vl output voltage 7v < v inl < 13.2v, v fb1 = v fb2 = v fbp = 1.9v, v fbn = 0.5v, i vl = 25ma 4.8 5.0 5.1 v vl undervoltage lockout threshold vl rising, 2.5% hysteresis 3.8 4.0 4.1 v reference ref output voltage no external load 1.98 2.00 2.02 v ref load regulation 0 < i ref < 50? 10 mv ref sink current ref in regulation 0 10 ? ref undervoltage lockout threshold rising edge, 200mv hysteresis 1.5 v max8728 low-cost, multiple-output power supply for lcd monitors/tvs _______________________________________________________________________________________ 3 electrical characteristics (continued) (circuit of figure 1, v in = v inl = v supp = 12v, v out1 = +3.3v, v src = 28v, gnd1 = gnd2 = gndp = gnd = 0, i ref = 0, t a = 0? to +85? . typical values are at t a = +25?, unless otherwise noted.) parameter conditions min typ max units step-down regulator out1 voltage in fixed mode v in = 7.0v to 13.2v, en = v cc , i load = 0.5a (note 1) 3.25 3.30 3.35 v fb1 regulation voltage in adjustable modes 20% to 35% duty cycle, en = v cc , i load = 0.5a (note 1) 1.97 2.00 2.03 v fb1 adjustable mode threshold voltage 0.10 0.15 0.20 v output voltage adjust range 2.0 3.6 v fixed mode, out1 falling 2.640 step-down regulator fault trip level adjustable mode, fb1 falling 1.536 1.600 1.664 v fb1 input leakage current v fb1 = 2.1v -100 +100 na low-frequency operation out1 threshold lx1 only 1.3 v fsel = gnd 250 fsel = v cc 167 low-frequency operation switching frequency lx1 only fsel = ref 83 khz dc load regulation 0 < i out1 < 2a, en = v cc 0.5 % dc line regulation 7v max8728 low-cost, multiple-output power supply for lcd monitors/tvs _______________________________________________________________________________________ 5 electrical characteristics (continued) (circuit of figure 1, v in = v inl = v supp = 12v, v out1 = +3.3v, v src = 28v, gnd1 = gnd2 = gndp = gnd = 0, i ref = 0, t a = 0? to +85? . typical values are at t a = +25?, unless otherwise noted.) parameter conditions min typ max units del, en discharge switch on-resistance shdn = low or fault tripped 20 fbn discharge switch on-resistance en = low or fault tripped 5 k positive gate-driver timing and control switches ctl input low voltage 0.6 v ctl input high voltage 2.0 v ctl input leakage current -1 +1 ? ctl-to-gon rising propagation delay 1k from drn to gnd, 1.5nf from gon to gnd 100 ns ctl-to-gon falling propagation delay 1k from drn to gnd, 1.5nf from gon to gnd 250 ns src input voltage range 38 v v mode = v ref , v del = v ctl = 3v 1.5 2.0 src input current v mode = v ref , v del = 3v, v ctl = 0 0.14 0.20 ma drn input current v mode = v ref , v drn = 8v, v del = 3v, v gon > v drn , v ctl = 0 01 a src switch on-resistance v mode = v ref , v del = v ctl = 3v 15 30 src switch saturation current v mode = v ref , v del = v ctl = 3v, v src - v gon > 5v 260 ma drn switch on-resistance v mode = v ref , v del = 3v, v ctl = 0, v gon = 28v, v thr = 1.4v 25 50 drn switch saturation current v mode = v ref , v del = 3v, v ctl = 0, v gon = 28v, v thr = 1.4v, v gon - v drn > 5v 100 ma mode switch on-resistance shdn = gnd 1 k mode current-source stop voltage threshold mode rising 1.2 1.4 1.6 v mode charge current operating mode 2, v mode = 0.7v 40 50 60 ? mode voltage threshold enabling drn switch control in mode 2 0.8 1.0 1.2 v thr to gon voltage gain 9.4 10.0 10.6 v/v fault detection duration to trigger fault 50 ms thermal shutdown threshold 15? typical hysteresis +160 ? switching-frequency selection fsel = v cc (1mhz) v cc - 0.4 fsel = ref (0.5mhz) 1.65 2.35 fsel input levels fsel = gnd (1.5mhz) 0.5 v fsel input current forced to v cc 10 ? max8728 low-cost, multiple-output power supply for lcd monitors/tvs 6 _______________________________________________________________________________________ electrical characteristics (circuit of figure 1, v in = v inl = v supp = 12v, v out1 = +3.3v, v src = 28v, gnd1 = gnd2 = gndp = gnd = 0, i ref = 0, t a = -40? to +85? .) (note 2) parameter conditions min typ max units general in, inl input voltage range for vl regulator operation 7.0 13.2 v in standby supply current v in = 7v to 13.2v, en = shdn = gnd 0.5 ma fsel = gnd 1175 1800 fsel = v cc 780 1150 switching frequency fsel = ref 400 610 khz vl regulator vl output voltage 7v < v inl < 13.2v, v fb1 = v fb2 = v fbp = 1.9v, v fbn = 0.5v, i vl = 25ma 4.8 5.1 v vl undervoltage lockout threshold vl rising, 2.5% hysteresis 3.8 4.1 v reference ref output voltage no external load 1.97 2.02 v ref load regulation 0 < i rfi < 50? 10 mv step-down regulator out1 voltage in fixed mode v in = 6.0v to 13.2v, en = v cc , i load = 0.5a (note 1) 3.23 3.35 v fb1 regulation voltage in adjustable mode 20% to 35% duty cycle, en = v cc , i out1 = 0.5a (note 1) 1.97 2.03 v fb1 adjustable-mode threshold voltage 0.10 0.20 v output voltage adjust range 2.0 3.6 v step-down regulator fault trip level adjustable mode, fb1 falling 1.536 1.664 v lx1-to-in switch on-resistance 550 m lx1-to-gnd1 switch on-resistance 840 positive current limit 2.3 3.1 a skip mode i max threshold en = gnd 0.45 0.75 a maximum duty cycle 70 85 % step-up regulator output voltage range v in 17 v maximum duty cycle 65 85 % fb2 regulation voltage fb2 = comp, c comp = 1nf 1.97 2.02 v lx2 current limit v fb2 = 1.8v, duty cycle is 25% 1.2 1.8 a lx2 on-resistance 1 max8728 low-cost, multiple-output power supply for lcd monitors/tvs _______________________________________________________________________________________ 7 note 1: when the inductor is in continuous conduction (en = v cc or heavy load), the output voltage has a dc regulation level lower than the error comparator threshold by 50% of the output voltage ripple. in discontinuous conduction (en = gnd with light load), the output voltage has a dc regulation level higher than the error comparator threshold by up to 50% of the output voltage ripple. note 2: specifications to -40? are guaranteed by design, not production tested. electrical characteristics (continued) (circuit of figure 1, v in = v inl = v supp = 12v, v out1 = +3.3v, v src = 28v, gnd1 = gnd2 = gndp = gnd = 0, i ref = 0, t a = -40? to +85? .) (note 2) parameter conditions min typ max units charge-pump regulators fbp regulation voltage 1.97 2.02 v fbn regulation voltage v ref - v fbn 1.71 1.78 v sequence control shdn input low voltage 0.4 v shdn input high voltage 2 v en turn-on threshold 0.95 1.10 v del turn-on threshold 0.95 1.10 v gate on voltage en = high v in - 6 v in - 4 v gate done threshold en = high, v gate _ done - v gate _ on 0v positive gate-driver timing and control switches ctl input low voltage 0.6 v ctl input high voltage 2.1 v src input voltage range 38 v v mode = v ref , v del = v ctl = 3v 2.3 src input current v mode = v ref , v del = 3v, v ctl = 0 0.2 ma src switch on-resistance v mode = v ref , v del = v ctl = 3v 30 drn switch on-resistance v mode = v ref , v del = 3v, v ctl = 0, v gon = 28v, v thr = 1.4v 50 mode current-source stop- voltage threshold mode rising 1.2 1.6 v mode voltage threshold enabling drn switch control in mode 2 0.8 1.2 v thr-to-gon voltage gain 9.4 10.6 v/v max8728 low-cost, multiple-output power supply for lcd monitors/tvs 8 _______________________________________________________________________________________ typical operating characteristics (circuit of figure 1. v in = v inl = v supp = 12v, av dd = 13.5v, v gon = 28v, v goff = -6v, v out1 = 3.3v, fsel = gnd, t a = +25?, unless otherwise noted.) step-down regulator efficiency vs. load current load current (a) efficiency (%) max8728 toc01 50 55 60 65 70 75 80 85 90 0.01 0.1 1 10 en = gnd en = vl normalized step-down regulator output voltage vs. load current load current (a) output voltage (v) max8728 toc02 3.10 3.15 3.20 3.25 3.30 3.35 3.40 0.01 0.1 1 10 en = gnd en = vl step-down regulator load transient response max8728toc03 0v 0v 0v load control, 5v/div lx1, 20v/div out1, ac, 100mv/div inductor current, 1a/div 4 s/div step-down regulator soft-start (heavy load) max8728toc04 0v 0v 0v v in , 5v/div out1, 2v/div inductor current, 1a/div 400 s/div step-up regulator efficiency vs. load current (measured at l1/c3 junction) load current (a) efficiency (%) max8728 toc05 60 65 70 75 80 85 90 95 100 0.001 0.01 0.1 1 10 normalized step-up regulator output voltage vs. load current load current (a) output voltage (v) max8728 toc06 13.30 13.35 13.40 13.45 13.50 13.55 13.60 0.01 0.1 1 10 max8728 low-cost, multiple-output power supply for lcd monitors/tvs _______________________________________________________________________________________ 9 typical operating characteristics (continued) (circuit of figure 1. v in = v inl = v supp = 12v, av dd = 13.5v, v gon = 28v, v goff = -6v, v out1 = 3.3v, fsel = gnd, t a = +25?, unless otherwise noted.) step-up regulator soft-start (heavy load) max8728toc07 0v 0v 0v en, 2v/div inductor current, 500ma/div 1ms/div av dd , 5v/div gate, 5v/div step-up regulator load transient response (100ma to 600ma) max8728toc08 0v 0v load control, 5v/div inductor current, 500ma/div av dd , ac, 200mv/div 10 s/div step-up regulator pulsed-load transient response (100ma to 1a) max8728toc09 0v 0v load control, 5v/div inductor current, 500ma/div av dd , ac, 100mv/div 4 s/div timer-delay overcurrent protection max8728toc10 0v vgon, 20v/div, 10k load inductor current, 1a/div 10ms/div v goff , 5v/div, avdd, 10k load out1, 2v/div, 2.5a overload av dd , 5v/div, 150 load switching frequency vs. input voltage input voltage (v) switching frequency (mhz) max8728 toc11 7 8 9 10 11 12 13 1.500 1.505 1.510 1.515 1.520 fsel = gnd normalized vl output voltage vs. vl current vl current (ma) vl voltage (v) max8728 toc12 0 10203040506070 4.80 4.85 4.90 4.95 5.00 5.05 en = gnd en = vl typical operating characteristics (continued) (circuit of figure 1. v in = v inl = v supp = 12v, av dd = 13.5v, v gon = 28v, v goff = -6v, v out1 = 3.3v, fsel = gnd, t a = +25?, unless otherwise noted.) max8728 low-cost, multiple-output power supply for lcd monitors/tvs 10 ______________________________________________________________________________________ positive charge-pump load transient response max8728toc16 src, ac, 200mv/div i src , 20ma/div 40 s/div negative charge-pump output voltage error vs. input voltage (v out = -6v) input voltage (v) output voltage error (%) max8728 toc17 7 8 9 10111213 -5 -4 -3 -2 -1 0 1 2 i out = 10ma i out = 100ma negative charge-pump output voltage error vs. load current load current (a) output voltage error (%) max8728 toc18 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 0.001 0.01 0.1 1 ref voltage vs. ref current ref current ( a) ref voltage (v) max8728 toc13 0 20406080100 1.994 1.995 1.996 1.997 1.998 1.999 2.000 positive charge-pump output voltage error vs. input voltage (v out = 28v) input voltage (v) output voltage error (%) max8728 toc14 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 i out = 10ma i out = 50ma positive charge-pump output voltage error vs. load current load current (a) output voltage error (%) max???? toc15 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 0.001 0.01 0.1 typical operating characteristics (continued) (circuit of figure 1. v in = v inl = v supp = 12v, av dd = 13.5v, v gon = 28v, v goff = -6v, v out1 = 3.3v, fsel = gnd, t a = +25?, unless otherwise noted.) max8728 low-cost, multiple-output power supply for lcd monitors/tvs ______________________________________________________________________________________ 11 negative charge-pump load transient response max8728toc19 v goff , ac, 50mv/div i vgoff , 50ma/div 40 s/div power-up sequence max8728toc20 2ms/div del, c del = 0.01 f 2v/div v gon , 20v/div, 10k load v goff , 5v/div, 10k load en, c en = 0.01 f, 2v/div av dd , 5v/div, 150 load out1, 5v/div, 100 load inl supply current vs. inl voltage inl voltage (v) inl supply current (ma) max8728 toc21 7 8 9 10111213 0 2 4 6 8 10 en = gnd en = gnd inl supply current vs. temperature temperature ( c) inl supply current (ma) max87228 toc22 -40 -15 10 35 60 85 0 1 2 3 4 5 6 7 en = vl en = gnd high-voltage switch control (mode 1) max8728toc23 2 s/div v ctl , 5v/div 0v 0v 0v v mode , 2v/div v gon , 10v/div high-voltage switch control (mode 2) max8728toc23 2 s/div v ctl , 5v/div 0v 0v 0v v mode , 2v/div v gon , 10v/div max8728 low-cost, multiple-output power supply for lcd monitors/tvs 12 ______________________________________________________________________________________ pin description pin name function 1 gnd1 step-down regulator and negative charge-pump power ground 2 out1 step-down regulator output sense input. out1 is the inverting input to the internal current-sense amplifier. connect out1 directly to the step-down regulator output. 3 drvn negative charge-pump regulator driver output. see the negative charge-pump regulator section for details. 4 ctl high-voltage switch-control block timing control input. see the high-voltage switch control section for 5 in step-down regulator and negative charge-pump regulator supply input 6 lx1 step-down regulator switching node. lx1 is the source of the internal high-side mosfet. connect the inductor and schottky catch diode to lx1 and minimize the trace area for low emi. 7 bst step-down regulator bootstrap pin. bst is the supply for the high-side mosfet gate driver. connect a 0.1? ceramic capacitor from bst to lx1. 8 inl 5v internal linear regulator and startup circuitry supply input. the input voltage range of inl is between +7.0v and +13.2v. connect a 0.22? ceramic capacitor between inl and gnd. place the capacitor close to the ic. 9vl 5v internal linear regulator output. vl powers the internal mosfet gate drivers and the control circuitry. bypass vl to gnd with a 1? ceramic capacitor. vl can provide up to 25ma external load current. 10 v cc internal reference supply input. connect v cc directly to vl. 11 shdn active-low shutdown control input. all outputs (except for ref and vl) are disabled and the gate pin goes high when shdn is low. 12 gnd analog ground 13 ref reference output. connect a 0.22? ceramic capacitor between ref and gnd. all regulator outputs are disabled until ref exceeds its uvlo threshold. 14 fbp positive charge-pump regulator feedback input. connect fbp to the center of a resistive voltage-divider between the positive output and gnd to set the positive charge-pump regulator output voltage. place the resistive voltage-divider close to fbp. 15 gndp positive charge-pump power ground 16 drvp positive charge-pump regulator driver output. see the positive charge-pump regulator section for details. 17 supp positive charge-pump regulator supply input. connect supp directly to in and bypass supp to gndp with a minimum 0.1? ceramic capacitor. 18 fsel frequency select pin. connect fsel to ref for 500khz operation. connect fsel to v cc for 1mhz operation. connect to gnd for 1.5mhz operation. 19 src high-voltage switch control block input. src is the source of the internal, high-voltage, p-channel mosfet. 20 gon high-voltage switch control block output. gon is the common junction of the internal high-voltage mosfets. gon is internally pulled to gnd through a 4ma internal current source when the switch control block is disabled. max8728 low-cost, multiple-output power supply for lcd monitors/tvs ______________________________________________________________________________________ 13 pin description (continued) pin name function 21 drn high-voltage switch control input. drn is the drain of the internal high-voltage p-channel mosfet connected to gon. see the high-voltage switch control section for details. 22 thr gon falling regulation adjustment input. connect thr to the center of a resistive voltage-divider between a reference supply and gnd to adjust the gon falling regulation set point. ctl and mode allow gon to disconnect from src and be discharged through drn; discharge stops when gon reaches 10 x v thr . see the high-voltage switch control section for details. 23 fb2 step-up regulator feedback input. connect fb2 to the center of a resistive voltage-divider between the step- up regulator output and gnd to set the step-up regulator output voltage. place the resistive voltage-divider close to fb2. 24 fbn negative charge-pump regulator feedback input. connect fbn to the center of a resistive voltage-divider between the negative output and ref to set the negative charge-pump regulator output voltage. place the resistive voltage-divider close to fbn. 25 gnd2 step-down regulator power ground 26 lx2 step-up regulator switching node. connect the inductor and the schottky diode to lx2 and minimize the trace area for low emi. 27 gate input mosfet gate-driver output. gate controls an external p-channel mosfet between the input voltage and the step-up regulator? inductor. the switch is off when the step-up regulator is turned off, so that the regulator? output discharges to ground. during startup, the step-up regulator? soft-start begins when v gate falls below the gate done threshold. 28 en enable input. pulling en high or leaving en unconnected enables the step-up regulator and the negative charge pump. connecting en to gnd disables the above blocks and puts the step-down regulator in skip mode. en sources 5? to allow a capacitor-controlled startup delay. 29 mode high-voltage switch-control block mode selection input and timing-adjustment input. see the high-voltage switch control section for details. 30 del positive charge-pump regulator and high-voltage switch-control delay input. connect a capacitor between del and gnd to set the delay time. a 5? current source charges c del . del is internally pulled to gnd through a 20 internal resistor in shutdown. 31 comp step-up regulator error amplifier compensation pin. see the loop compensation section for details. 32 fb1 step-down regulator feedback input. connect fb1 to the center of a resistive voltage-divider between the step-down regulator output and gnd to set the step-down regulator output voltage. ?p exposed pad. connect the exposed backside pad to gnd and provide adequate thermal path to cool the ic. see the pc board layout and grounding section. max8728 low-cost, multiple-output power supply for lcd monitors/tvs 14 ______________________________________________________________________________________ max8728 av dd 13.5v/500ma gate p1 d1 d6 inl v in 10.8v to 13.2v out1 3.3v/2a v goff -6v/150ma d5 d4 in 26 25 23 31 4 22 29 21 20 19 14 15 lx1 out1 fb1 en shdn gnd1 gnd fbp gndp src ref gon drn mode thr comp c1 10 f 16v c9 22 f 6.3v c2 0.22 f l1 6.4 h c3 10 f 16v c24 1000pf c4 10 f 16v c7 1 f c10 0.1 f r9 44.2k 1% r10 158k 1% r3 160k r4 127k 1% r5 22.1k 1% r12 10.0k , 1% r11 6.49k , 1% r6 2k r1 115k 1% r2 20k 1% r7 287k 1% r8 22.1k 1% from tcon ctl fb2 lx2 5 8 v l v cc l2 2.6 h 910 27 7 6 1 2 32 11 28 del c11 0.1 f c12 0.22 f c25 47pf c14 220pf c15 1 f c28 10pf c13 100pf c16 0.1 f c17 1 f c21 0.1 f c20 0.1 f c18 0.1 f c19 0.1 f 30 13 12 24 3 18 17 16 d2 c8 0.1 f c5 10 f 16v c6 10 f 16v gnd2 fbn drvn supp fsel drvp bst v gon 28v/50ma on/off d3 vin figure 1. typical operating circuit max8728 low-cost, multiple-output power supply for lcd monitors/tvs ______________________________________________________________________________________ 15 typical operating circuit the typical operating circuit (figure 1) of the max8728 is a complete power-supply system for tft lcd panels in monitors and tvs. the circuit generates a +3.3v logic supply, a +13.5v source driver supply, a +28v positive gate driver supply, and a -6v negative gate driver supply from a 12v ?0% input supply and operates at 1.5mhz. table 1 lists some selected components and table 2 lists the contact information for component suppliers. detailed description the max8728 is a multiple-output power supply designed primarily for tft lcd panels used in monitors and tvs. it contains a step-down switching regulator to generate the logic supply rail, a step-up switching regu- lator to generate the source driver supply, and two charge-pump regulators to generate the gate-driver supplies. each regulator features adjustable output volt- age, digital soft-start, and timer-delayed fault protection. both the step-down and step-up regulators use fixed- frequency current-mode control architectures. the two switching regulators are 180 out of phase to minimize the input ripple. the internal oscillator offers three pin- selectable frequency options (500khz/1mhz/1.5mhz) allowing users to optimize their designs based on the specific application requirements. in addition, the max8728 features a high-voltage switch-control block, an internal 5v linear regulator, a 2v reference output, well-defined power-up and power-down sequences, and thermal-overload protection. figure 2 shows the max8728 functional diagram. step-down regulator the step-down regulator consists of an internal n-chan- nel mosfet with gate driver, a lossless current-sense network, a current-limit comparator, and a pwm con- troller block. the external power stage consists of a schottky diode rectifier, an inductor, and output capac- itors. the output voltage is regulated by changing the duty cycle of the high-side mosfet. a bootstrap circuit that uses a 0.1? flying capacitor between lx1 and bst provides the supply voltage for the high-side gate driver. although the max8728 also includes a 25 (typ) low-side mosfet, this switch is used to charge the bootstrap capacitor during startup and maintains fixed- frequency operation at light load and cannot be used as a synchronous rectifier. an external schottky diode (d2 in figure 1) is always required. pwm controller block the heart of the pwm control block is a multi-input, open-loop comparator that sums three signals: the out- put voltage signal with respect to the reference voltage, the current-sense signal, and the slope compensation. the pwm controller is a direct-summing type, lacking a traditional error amplifier and the phase shift associated with it. this direct-summing configuration approaches ideal cycle-by-cycle control over the output voltage. when en is high or floating, the controller always oper- ates in fixed-frequency pwm mode. each pulse from the oscillator sets the main pwm latch that turns on the high-side switch until the pwm comparator changes state. as the high-side switch turns off, the low-side switch turns on. the low-side switch stays on until the beginning of the next clock cycle. when en is low, the controller operates in skip mode. the skip mode dramatically improves light-load effi- ciency by reducing the effective frequency, which reduces switching losses. it keeps the actual peak inductor current at about 0.8a in an active cycle, allow- ing subsequent cycles to be skipped. skip mode tran- sitions seamlessly to fixed-frequency pwm operation as load current increases. table 1. component list (1.5mhz) designation description c1, c3, c4, c5, c6 10? ?0%, 16v x5r ceramic capacitors (1206) tdk c3216x5r1c106m d1, d2 3a, 30v schottky diode (m-flat) toshiba cms02 (top mark s2) d3, d4, d5 220ma, 100v dual diode (sot23) fairchild mmbd4148se (top mark d4) l1 6.4?, 1.5adc inductor sumida cdrh6d12-6r4 l2 2.6?, 2.6adc inductor sumida cdrh6d12-2r6 p1 2.4a, -20v p-channel mosfet (3-pin supersot) fairchild fdn304p (top mark 304) table 2. component suppliers supplier phone fairchild semiconductor 408-822-2000 sumida 847-545-6700 tdk 847-803-6100 toshiba 949-455-2000 max8728 low-cost, multiple-output power supply for lcd monitors/tvs 16 ______________________________________________________________________________________ v l in bst gate inl lx2 gnd2 fb2 comp lx1 gnd1 out1 fb1 shdn en del ref gnd fbn fsel drvn supp ctl thr mode drn gon src fbp drvp v in out1 v cc v l v l v goff av dd v gon from tcon on/off lcd enable in step-down regulator sequence control v l regulator step-up regulator thermal shutdown oscillator fault logic and timer reference negative regulator switch control block positive regulator max8728 vin figure 2. functional diagram max8728 low-cost, multiple-output power supply for lcd monitors/tvs ______________________________________________________________________________________ 17 current limiting and lossless current sensing the current-limit circuit turns off the high-side mosfet switch whenever the voltage across the high-side mosfet exceeds an internal threshold corresponding to the actual current limit of 2.8a ?0%. for current-mode control, an internal lossless sense network derives a current-sense signal from the induc- tor dc resistance. the time constant of the current- sense network is not required to match the time constant of the inductor and has been chosen to pro- vide sufficient current-ramp signal for stable operation at each operating frequency. the current-sense signal is ac-coupled into the pwm comparator, eliminating most dc output voltage variation with load current. low-frequency operation the step-down regulator of the max8728 enters into low-frequency operating mode if the voltage on out1 is below 1.3v. in the low-frequency mode, the switching frequency of the step-down regulator is 1/6 the oscilla- tor frequency. this feature prevents potentially uncon- trolled inductor current if out1 is overloaded or shorted to ground. soft-start and fault protection the step-down regulator includes a 7-bit soft-start dac that steps the internal reference voltage from zero to 2v in 128 steps. the soft-start period is 3ms (typ) and fb1 fault detection is disabled during this period. the soft- start feature effectively limits the inrush current during startup (see the step-down regulator soft-start waveforms in the typical operating characteristics ). the max8728 monitors out1 (fixed-output mode) or fb1 (adjustable-output mode) for undervoltage condi- tions. if the voltage is continuously below 80% (typ) of the nominal regulation point for approximately 50ms, the max8728 sets a fault latch, shutting down all out- puts except vl and ref. step-up regulator the step-up regulator employs a current-mode, fixed- frequency pwm architecture to maximize loop band- width and provide fast transient response to pulsed loads typical of tft lcd panel source drivers. the inte- grated mosfet and the built-in digital soft-start function reduce the number of external components required while controlling inrush currents. the output voltage can be set from v in to 28v with an external resistive voltage- divider. the regulator controls the output voltage and the power delivered to the output by modulating the duty cycle of the internal power mosfet in each switching cycle. pwm controller block an error amplifier compares the signal at fb2 to 2.0v and changes the comp output. the voltage at comp sets the peak inductor current. as the load varies, the error amplifier sources or sinks current to the comp output accordingly to produce the inductor peak cur- rent necessary to service the load. to maintain stability at high duty cycles, a slope-compensation signal is summed with the current-sense signal. on the rising edge of the internal clock, the controller sets a flip-flop, turning on the n-channel mosfet and applying the input voltage across the inductor. the cur- rent through the inductor ramps up linearly, storing energy in its magnetic field. once the sum of the cur- rent-feedback signal and the slope compensation exceed the comp voltage, the controller resets the flip- flop and turns off the mosfet. since the inductor cur- rent is continuous, a transverse potential develops across the inductor that turns on the diode (d1). the voltage across the inductor then becomes the differ- ence between the output voltage and the input voltage. this discharge condition forces the current through the inductor to ramp back down, transferring the energy stored in the magnetic field to the output capacitor and the load. the mosfet remains off for the rest of the clock cycle. input switch control the gate pin of the max8728 controls an optional external p-channel mosfet between the input supply and the inductor of the step-up regulator. this function disconnects the step-up regulator from the input supply and allows the regulator output to discharge to ground when the step-up regulator is disabled. when en is low, gate is internally pulled up to the input supply through a 1k resistor. once en and shdn are high and the negative charge-pump regulator is in regulation, the max8728 starts pulling down gate with an 11? inter- nal current source. the external p-channel mosfet turns on and connects the input supply to the step-regu- lator when v gate falls below the turn-on threshold of the mosfet. when v gate reaches v in - 4v, the step-up regulator is enabled and initiates a soft-start routine. v gate continues to fall until it reaches v in - 5v. soft-start and fault protection the step-up regulator achieves soft-start by linearly ramping up its internal current limit. the soft-start termi- nates when the output reaches regulation or the full current limit has been reached. the current limit rises from zero to the full current limit in approximately 3ms. max8728 low-cost, multiple-output power supply for lcd monitors/tvs 18 ______________________________________________________________________________________ the soft-start feature effectively limits the inrush current during startup (see the step-up regulator soft-start waveforms in the typical operating characteristics ). the max8728 monitors fb2 for undervoltage condi- tions. if the voltage is continuously below 90% of the nominal regulation point for approximately 50ms, the max8728 sets a fault latch, shutting down all outputs except vl, ref, and the step-down regulator. positive charge-pump regulator the positive charge-pump regulator is typically used to generate the positive supply rail for the tft lcd gate- driver ics. the output voltage is set with an external resistive voltage-divider from its output to gnd with the midpoint connected to fbp. the number of charge- pump stages and the setting of the feedback divider determine the output voltage of the positive charge- pump regulator. the charge pump includes a high- side, p-channel mosfet (p1) and a low-side, n-channel mosfet (n1) to control the power transfer as shown in figure 3. the error comparator compares the feedback signal (fbp) with a 2.0v internal reference. if the feedback signal is below the reference, the charge-pump regula- tor turns on p1 and turns off n1 when the rising edge of the oscillator clock arrives, level shifting the flying capacitors (c18 and c19) by v supp volts. if the result- ing voltage on c18 and c19 is greater than their asso- ciated reservoir capacitors (c20 and c15), charge flows until the diode connecting each flying capacitor to its reservoir capacitor turns off. the falling edge of the oscillator clock turns off p1 and turns on n1, charging the flying capacitors (c18 and c19) through the diodes that connect them to the reservoir capacitors (c21 and c20). if the feedback signal is above the reference when the rising edge of the oscillator comes, the regula- tor ignores this clock edge and keeps n1 on and p1 off. the positive charge-pump regulator? startup can be delayed by connecting an external capacitor from del to gnd. an internal constant-current source begins charg- ing the del capacitor when en and shdn are logic high, the negative charge pump reaches regulation, and gate has gone low. when the del voltage exceeds v ref /2, the positive charge-pump regulator is enabled. each time it is enabled, the positive charge-pump regulator goes through a soft-start routine by ramping up its internal ref- erence voltage from 0 to 2v in 128 steps. the soft-start period is 3ms (typ) and fbp fault detection is disabled during this period. the soft-start feature effectively limits the inrush current during startup. the max8728 also monitors the fbp voltage for undervoltage conditions. if vfbp is continuously below 80% of its nominal regulation point for approximately 50ms, the max8728 sets a fault latch, shutting down all outputs except vl, ref, and the step-down regulator. level shift ref q clk d osc ff error comparator p1 n1 drvp c21 c20 c15 c28 c19 c18 gndp supp fbp positive charge-pump regulator input supply output max8728 figure 3. positive charge-pump regulator block diagram max8728 low-cost, multiple-output power supply for lcd monitors/tvs ______________________________________________________________________________________ 19 negative charge-pump regulator the negative charge-pump regulator is typically used to generate the negative supply rail for the tft lcd gate- driver ics. the output voltage is set with an external resistive voltage-divider from its output to ref with the midpoint connected to fbn. the number of charge- pump stages and the setting of the feedback-divider determine the output of the negative charge-pump regu- lator. the charge-pump controller includes a high-side, p-channel mosfet (p2) and a low-side, n-channel mosfet (n2) to control the power transfer as shown in figure 4. the error comparator compares the feedback signal (fbn) with a 250mv internal reference. if the feedback signal is above the reference, the charge-pump regula- tor turns on n2 and turns off p2 when the rising edge of the oscillator clock arrives, level shifting the flying capacitor (c16). the falling edge of the oscillator clock turns off n2 and turns on p2, charging the flying capac- itor (c16) through the diode that connects it to the reservoir capacitor (c1). if the feedback signal is below the reference (output is in regulation) when the rising edge of the oscillator comes, the regulator ignores this clock edge and keeps p2 on and n2 off. the negative charge-pump regulator is enabled when shdn and en are logic high and the step-down regula- tor reaches regulation. each time it is enabled, the neg- ative charge-pump regulator goes through a soft-start routine by ramping down its internal reference voltage from 2v to 250mv in 128 steps. the soft-start period is 3ms (typ) and fbn fault detection is disabled during this period. the soft-start feature effectively limits the inrush current during startup. the max8728 also moni- tors the fbn voltage for undervoltage conditions. if v fbn is continuously above 600mv for approximately 50ms, the max8728 sets a fault latch, shutting down all outputs except vl, ref, and the step-down regulator. high-voltage switch control the max8728? high-voltage switch control block (figure 5) consists of two high-voltage, p-channel mosfets: q1, between src and gon and q2, between gon and drn. the switch control block is enabled when v del goes above v ref / 2. q1 and q2 are controlled by ctl and mode. there are two differ- ent modes of operation (see the typical operating characteristics section). activate the first mode by connecting mode to ref. when ctl is logic high, q1 turns on and q2 turns off, connecting gon to src. when ctl is logic low, q1 0.25v q clk d osc ff error comparator p2 n2 drvn gnd1 in fbn negative charge-pump regulator input supply output ref max8728 c17 c16 c1 figure 4. negative charge-pump regulator block diagram max8728 low-cost, multiple-output power supply for lcd monitors/tvs 20 ______________________________________________________________________________________ turns off and q2 turns on, connecting gon to drn. gon can then be discharged through a resistor con- nected between drn and gnd or av dd . q2 turns off and stops discharging gon when v gon reaches 10 times the voltage on thr. when v mode is less than 0.9 x v ref , the switch control block works in the second mode. the rising edge of v ctl turns on q1 and turns off q2, connecting gon to src. an internal n-channel mosfet q3 between mode and gnd is also turned on to discharge an external capacitor between mode and gnd. the falling edge of vctl turns off q3, and an internal 50? current source starts charging the mode capacitor. once v mode exceeds 0.5 x v ref , the switch control block turns off q1 and turns on q2, connecting gon to drn. gon can then be discharged through a resistor connected between drn and gnd or av dd . q2 turns off and stops discharging gon when v gon reaches 10 times the voltage on thr. when the lcd is shut down or in a fault state, the switch control block is disabled, dzl is held low, and gon is discharged to gnd through an internal 4ma current source. if the drn resistor connects drn to av dd or another voltage above ground, the q2 body diode conducts. to prevent the body diode conduc- tion, an external diode must be added in series with the drn resistor (d6 in figure 1). during startup, the 4ma current source and q4 are released when gate reach- es the gate done threshold. linear regulator (vl) the max8728 includes an internal linear regulator. inl is the input of the linear regulator. the input voltage range is between 7v and 13.2v. the output voltage is set to 5v. the regulator powers the internal mosfet drivers, pwm controllers, charge-pump regulators, and logic circuitry. the total external load capability is 25ma. bypass vl to gnd with a minimum 1? ceramic capacitor. reference voltage (ref) the reference output is nominally 2v, and can source at least 50? (see the typical operating characteristics section). v cc is the input of the internal reference block. bypass ref with a 0.22? ceramic capacitor connected between ref and gnd. frequency selection (fsel) the step-down regulator and step-up regulator use the same internal oscillator. the fsel input selects the switching frequency. table 3 shows the switching fre- quency based on the fsel connection. high-frequency (1.5mhz) operation optimizes the application for the smallest component size, trading off efficiency due to higher switching losses. low-frequency (500khz) oper- ation offers the best overall efficiency at the expense of component size and board space. to reduce the input rms current, the step-down regu- lator and the step-up regulator operate 180 out of phase from each other. the feature allows the use of less input capacitance. power-up sequence the step-down regulator starts up when the max8728? internal reference voltage (ref) is above its undervolt- age lockout (uvlo) threshold and shdn is logic high. the fb1 fault-detection circuit is enabled after the step- down regulator reaches regulation. the negative charge-pump regulator starts up when both en and shdn are logic high and ref is above its uvlo thresh- old. once the negative charge-pump regulator output is in regulation, the max8728 enables the fbn fault-detec- tion circuit and the input-switch control block, which starts pulling down gate with a 11? internal current source. the external p-channel mosfet turns on and connects the input supply to the step-up regulator when v gate falls below the turn-on threshold of the mosfet. when v gate reaches the gate done threshold, the max8728 enables the step-up regulator and the posi- tive charge-pump adjustable delay block. the fb2 fault-detection circuit is enabled after the step-up regu- lator reaches regulation. the delay block charges the del capacitor with an internal 5? current source and v del rises linearly. when v del exceeds 1v (typ), the max8728 enables the positive charge-pump regulator and the high-voltage switch control block. the fbp fault detection is enabled after the positive charge-pump regulator reaches regulation. power-down control the max8728 disables the step-up regulator, positive charge-pump regulator, negative charge-pump regula- tor, input switch control block, delay block, and high- voltage switch control block when en or shdn is logic low, or when any fault latch is set. the step-down regu- lator is disabled only when shdn is logic low, the step- down fault latch is set, or during thermal overload. table 3. frequency selection fsel switching frequency (khz) gnd 1500 vcc 1000 ref 500 max8728 low-cost, multiple-output power supply for lcd monitors/tvs ______________________________________________________________________________________ 21 mode ctl del src gon drn 0.5 x v ref gate done 5 a ref q1 4ma q4 q3 r 4r 5r 50 a switch control thr 9r 1k r ref 1k shdn fault en max8728 q2 figure 5. switch-control functional diagram max8728 low-cost, multiple-output power supply for lcd monitors/tvs 22 ______________________________________________________________________________________ fault protection during steady-state operation, if any output of the four regulators (step-down regulator, step-up regulator, positive charge-pump regulator, and negative charge- pump regulator) does not exceed its respective fault- detection threshold, the max8728 activates an internal fault timer. if any condition or the combination of condi- tions indicates a continuous fault for the fault-timer duration (50ms typ), the max8728 sets a fault latch. if the fault is caused by the step-up regulator or one of the charge pumps (lcd fault), the max8728 shuts down all the outputs except vl, ref, and the step- down regulator. once the fault condition is removed, toggle en or shdn , or cycle the input voltage to clear the lcd fault latch and restart the lcd supplies. if the fault is caused by the step-down regulator, the max8728 shuts down all the outputs except vl and ref. once the fault condition is removed, toggle shdn or cycle the input voltage to clear the step-down fault latch and restart the supplies. thermal-overload protection the thermal-overload protection prevents excessive power dissipation from overheating the max8728. when the junction temperature exceeds t j = +160?, a thermal sensor immediately activates the fault protec- tion, which shuts down all the outputs except the refer- ence, allowing the device to cool down. once the device cools down by approximately 15?, the max8728 automatically restarts all the supplies. the thermal-overload protection protects the controller in the event of fault conditions. for continuous opera- tion, do not exceed the absolute maximum junction temperature rating of t j = +150?. design procedure step-down regulator design inductor selection three key inductor parameters must be specified: inductance value (l), peak current (i peak ), and dc resistance (r dc ). the following equation includes a constant, lir, which is the ratio of peak-to-peak induc- tor ripple current to dc load current. a higher lir value allows smaller inductance, but results in higher losses and higher ripple. a good compromise between size and losses is typically found at a 30% ripple-current to load-current ratio (lir = 0.3), which corresponds to a peak inductor current 1.15 times the dc load current: where i out1(max) is the maximum dc load current, and the switching frequency f sw is 1.5mhz when fsel is tied to gnd, 1mhz when fsel is tied to v cc , and 500khz when fsel is tied to ref. the exact inductor value is not critical and can be adjusted to make trade- offs among size, cost, and efficiency. lower inductor values minimize size and cost, but they also increase the output ripple and reduce the efficiency due to high- er peak currents. on the other hand, higher inductor values increase efficiency, but at some point resistive losses due to extra turns of wire will exceed the benefit gained from lower ac current levels. the inductor? saturation current must exceed the peak inductor current. the peak current can be calculated by: the inductor? dc resistance should be low for good efficiency. find a low-loss inductor having the lowest possible dc resistance that fits in the allotted dimen- sions. ferrite cores are usually the best choice, espe- cially at the higher frequency settings. shielded-core geometries help keep noise, emi, and switching wave- form jitter low. input capacitors the input filter capacitors reduce peak currents drawn from the power source and reduce noise and voltage ripple on the input caused by the regulator? switching. they are usually selected according to input ripple cur- rent requirements and voltage rating, rather than capacitance value. the input voltage and load current determine the rms input ripple current (i rms ): the worst case is i rms = 0.5 x i out1 , which occurs at v in = 2 x v out1 . for most applications, ceramic capacitors are used because of their high ripple current and surge-current capabilities. for optimal circuit long-term reliability, choose an input capacitor that exhibits less than +10? temperature rise at the rms input current correspond- ing to the maximum load current. ii vvv v rms out out in out in = ? () 1 11 i vvv fl v ii i out ripple out in out sw out in out peak out max out ripple 1 11 1 11 1 2 _ _() _ = ? () =+ l vvv v f i lir out out in out in sw out max 1 11 1 () = ? () max8728 low-cost, multiple-output power supply for lcd monitors/tvs ______________________________________________________________________________________ 23 output-capacitor selection since the max8728? step-down regulator is internally compensated, it is stable with any reasonable amount of output capacitance. however, the actual capaci- tance and equivalent series resistance (esr) affect the regulator? output ripple voltage and transient response. the rest of this section deals with how to determine the output capacitance and esr needs according to the ripple voltage and load-transient requirements. the output voltage ripple has two components: varia- tions in the charge stored in the output capacitor, and the voltage drop across the capacitor? esr caused by the current into and out of the capacitor: where i out1 _ ripple is defined in the step-down regulator, inductor selection section, c out1 is output capacitance, and r esr _ out1 is the esr of output capacitor c out1 . in figure 1? circuit, the inductor rip- ple current is 0.6a. if the voltage ripple requirement of figure 1? circuit is ?% of the 3.3v output, then the total peak-to-peak ripple voltage should be less than 66mv. assuming that the esr ripple and the capacitive ripple each should be less than 50% of the total peak- to-peak ripple, then the esr should be less than 55m and the output capacitance should be more than 1.5? to meet the total ripple requirement. a 22? capacitor with esr (including pc board trace resistance) of 10m is selected for the standard application circuit in figure 1, which easily meets the voltage-ripple requirement. the step-down regulator? output capacitor and esr also affect the voltage undershoot and overshoot when the load steps up and down abruptly. the undershoot and overshoot also have two components: the voltage steps caused by esr and voltage sag and soar due to the finite capacitance and inductor slew rate. use the following formulae to check if the esr is low enough and the output capacitance is large enough to prevent excessive soar and sag. the amplitude of the esr step is a function of the load step and the esr of the output capacitor: v out1_esr_step = i out1 x r esr_out1 the amplitude of the capacitive sag is a function of the load step, the output capacitor value, the inductor value, the input-to-output voltage differential, and the maximum duty cycle: the amplitude of the capacitive soar is a function of the load step, the output capacitor value, the inductor value and the output voltage: given the component values in the circuit of figure 1, during a 2a step-load transient, the voltage step due to capacitor esr is negligible. the voltage sag and soar are 40.2mv and 71.6mv, respectively. rectifier diode the max8728? high switching frequency demands a high-speed rectifier. schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. in general, a 2a schottky diode works well in the max8728? step-down regulator. output-voltage selection connect a resistive voltage-divider between out1 and gnd with the center tap connected to fb1 to adjust the output voltage. choose r12 (resistance from fb1 to gnd) to be between 5k and 50k , and solve for r11 (resistance from out1 to fb1) using the equation: where v fb1 = 2v, and v out1 may vary from 2v to 3.6v. connecting a small capacitor (e.g., 47pf) between fb1 and gnd reduces fb1 noise sensitivity. step-up regulator design inductor selection the inductance value, peak-current rating, and series resistance are factors to consider when selecting the step-up inductor. these factors influence the convert- er? efficiency, maximum output load capability, tran- sient response time, and output voltage ripple. physical size and cost are also important factors to be consid- ered. the maximum output current, input voltage, output volt- age, and switching frequency determine the inductor value. very high inductance values minimize the cur- rr v v out fb 11 12 1 1 1 = ? ? ? ? ? ? ? v li cv out soar out out out out 1 11 2 11 2 _ () = v li cv dv out sag out out out in min max out 1 11 2 11 2 _ () () = ? () vv v vir v i cf out ripple out ripple esr out ripple c out ripple esr out ripple esr out out ripple c out ripple out sw 11 1 11 1 1 1 1 8 __()_() _() _ _ _() _ =+ = = max8728 low-cost, multiple-output power supply for lcd monitors/tvs 24 ______________________________________________________________________________________ rent ripple and therefore reduce the peak current, which decreases core losses in the inductor and i 2 r losses in the entire power path. however, large induc- tor values also require more energy storage and more turns of wire, which increase physical size and can increase i 2 r losses in the inductor. low inductance val- ues decrease the physical size but increase the current ripple and peak current. finding the best inductor involves choosing the best compromise between circuit efficiency, inductor size, and cost. the equations used here include a constant lir, which is the ratio of the inductor peak-to-peak ripple current to the average dc inductor current at the full load cur- rent. the best trade-off between inductor size and cir- cuit efficiency for step-up regulators generally has an lir between 0.2 and 0.5. however, depending on the ac characteristics of the inductor core material and ratio of inductor resistance to other power-path resis- tances, the best lir can shift up or down. if the induc- tor resistance is relatively high, more ripple can be accepted to reduce the number of turns required and increase the wire diameter. if the inductor resistance is relatively low, increasing inductance to lower the peak current can decrease losses throughout the power path. if extremely thin, high-resistance inductors are used, as is common for lcd panel applications, the best lir can increase to between 0.5 and 1.0. once a physical inductor is chosen, higher and lower values of the inductor should be evaluated for efficien- cy improvements in typical operating regions. calculate the approximate inductor value using the typ- ical input voltage (v in ), the maximum output current (i avdd(max) ), the expected efficiency ( typ ) taken from an appropriate curve in the typical operating characteristics , and an estimate of lir based on the above discussion: choose an available inductor value from an appropriate inductor family. calculate the maximum dc input cur- rent at the minimum input voltage v in(min) using con- servation of energy and the expected efficiency at that operating point ( min ) taken from an appropriate curve in the typical operating characteristics : calculate the ripple current at that operating point and the peak current required for the inductor: the inductor? saturation current rating and the max8728? lx2 current limit should exceed i avdd _ peak and the inductor? dc current rating should exceed i in(dc,max) . for good efficiency, choose an inductor with less than 0.1 series resistance. considering the typical operating circuit in figure 1, the maximum load current (i avdd(max) ) is 500ma with a 13.5v output and a typical input voltage of 12v. choosing an lir of 0.3 and estimating efficiency of 95% at this operating point: using the circuit? minimum input voltage (10.8v) and estimating efficiency of 90% at that operating point: the ripple current and the peak current are: output-capacitor selection the total output voltage ripple has two components: the capacitive ripple caused by the charging and dis- charging of the output capacitance, and the ohmic rip- ple due to the capacitor? esr: where i avdd _ peak is the peak-inductor current (see the inductor selection section). for ceramic capacitors, the vv v v i c vv vxf and vixr avdd ripple avdd ripple c avdd ripple esr avdd ripple c avdd avdd avdd in avdd sw avdd ripple esr avdd peak esr __()_() _() _() _ _ , =+ ? ? ? ? ? ? ? i vvv h v mhz a ia a a ripple peak . . . . . . . . . . = ? () =+ 10 8 13 5 10 8 6 4 13 5 1 5 023 069 023 2 081 i av v a in dc max (, ) . . . . . = 0 5 13 5 10 8 0 9 069 l v v vv a mhz h avdd . . .. . . . = ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 12 13 5 13 5 12 05 15 095 05 64 2 i vvv lvf ii i avdd ripple in min avdd in min avdd avdd sw avdd peak in dc max avdd ripple _ () () _(,) _ = ? () =+ 2 i iv v in dc max avdd max avdd in min min (, ) () () = l v v vv i f lir avdd in avdd avdd in avdd max sw typ () = ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 2 max8728 low-cost, multiple-output power supply for lcd monitors/tvs ______________________________________________________________________________________ 25 output voltage ripple is typically dominated by v avdd _ ripple(c) . the voltage rating and temperature characteristics of the output capacitor must also be considered. input-capacitor selection the input capacitor reduces the current peaks drawn from the input supply and reduces noise injection into the ic. two 10? ceramic capacitors are used in the typical applications circuit (figure 1) because of the high-source impedance seen in typical lab setups. actual applications usually have much lower source impedance since the step-up regulator often runs directly from the output of another regulated supply. typically, the input capacitance can be reduced below the values used in the typical operating circuit . rectifier diode the max8728? high-switching frequency demands a high-speed rectifier. schottky diodes are recommend- ed for most applications because of their fast recovery time and low forward voltage. in general, a 1a to 2a schottky diode complements the internal mosfet well. output-voltage selection the output voltage of the step-up regulator is adjusted by connecting a resistive voltage-divider from the out- put (v avdd ) to gnd with the center tap connected to fb2 (see figure 1). select r2 in the 10k to 50k range. calculate r1 with the following equation: where v fb2 , the step-up regulator? feedback set point, is 2.0v. place r1 and r2 close to the ic. loop compensation choose r comp (r3 in figure 1) to set the high-frequen- cy integrator gain for fast-transient response. choose c comp (c13 in figure 1) to set the integrator zero to maintain loop stability. for low-esr output capacitors, use the following equa- tions to obtain stable performance and good transient response: to further optimize transient response, vary r comp in 20% steps and c comp in 50% steps while observing transient response waveforms. charge-pump regulators selecting the number of charge-pump stages for highest efficiency, always choose the lowest num- ber of charge-pump stages that meet the output requirement. the number of positive charge-pump stages is given by: where n pos is the number of positive charge-pump stages, v gon is the output of the positive charge-pump regulator, i gon is the positive charge-pump output cur- rent, v supp is the supply voltage of the charge-pump regulators, v d is the forward voltage drop of the charge-pump diode, and r eff is the effective output resistance of the charge-pump switches (10 typ.) the number of negative charge-pump stages is given by: where n neg is the number of negative charge-pump stages, v goff is the output of the negative charge- pump regulator, and i goff is the negative charge- pump output current. the above equations assume that the flying capacitors are large enough to not further limit the output current. flying capacitors increasing the flying capacitor (c x ) value lowers the effective source impedance and increases the output current capability. increasing the capacitance indefi- nitely has a negligible effect on output current capabili- ty because the internal switch resistance and the diode impedance place a lower limit on the source imped- ance. a 0.1? ceramic capacitor works well, except in cases of low frequency, low headroom, and high cur- rent. the flying capacitor? voltage rating must exceed the following: v cx > n x v supp where n is the stage number in which the flying capaci- tor appears. n v vxvixr neg goff supp d goff eff ()( ) = ? ?? 2 n vv vvixr pos gon supp supp d gon eff ( ) ( ) = ? ? ? 2 r vv c li c vc ir comp in avdd avdd avdd avdd max comp avdd avdd avdd max comp () () 250 20 rr v v avdd fb 12 1 2 = ? ? ? ? ? ? ? max8728 low-cost, multiple-output power supply for lcd monitors/tvs 26 ______________________________________________________________________________________ charge-pump output capacitor decreasing the flying capacitance reduces the output ripple. increasing the output capacitance reduces the output ripple and improves the transient response. use the following equations to approximate the output ripple: where v out_pos is the positive charge-pump output voltage, c x_pos is the flying capacitor of the positive charge pump, c out_pos is the output capacitor of the positive charge-pump, v out_neg is the negative charge-pump output voltage, cx_neg is the flying capacitor of the negative charge pump, and c out_neg is the output capacitor of the negative charge pump. output-voltage selection adjust the positive charge-pump regulator? output volt- age by connecting a resistive voltage-divider from src to gnd with the center tap connected to fbp (figure 1). select the lower resistor of divider r7 in the 10k to 30k range. calculate upper resistor r8 with the follow- ing equation: where v fbp = 2v (typ). adding a small capacitor (e.g., 10pf) across r7 reduces pulse grouping and output noise. adjust the negative charge-pump regulator? output voltage by connecting a resistive voltage-divider from v goff to ref with the center tap connected to fbn (figure 1). select r9 in the 35k to 68k range. calculate r10 with the following equation: where v fbn = 250mv, v ref = 12v. note that ref can only source up to 50?; using a resistor less than 35k for r9 results in higher bias current than ref can supply. pc board layout and grounding careful pc board layout is important for proper operation. use the following guidelines for good pc board layout: 1) minimize the area of respective high-current loops by placing each dc-dc converter? inductor, diode, and output capacitors near its input capacitors and its lx_ and gnd_ pins. for the step-down regulator, the high-current input loop goes from the positive terminal of the input capacitor to the ic? in pin, out of lx1, to the inductor, to the positive terminals of the output capacitors, reconnecting the output capacitor and input capacitor ground terminals. the high-current output loop is from the inductor to the positive terminals of the output capacitors, to the negative terminals of the output capacitors, and to the schottky diode (d2). for the step-up regulator, the high-current input loop goes from the positive terminal of the input capacitor to the inductor, to the ic? lx2 pin, out of gnd2, and to the input capaci- tor? negative terminal. the high-current output loop is from the positive terminal of the input capacitor to the inductor, to the output diode (d1), to the positive terminal of the output capacitors, reconnecting between the output capacitor and input capacitor ground terminals. connect these loop components with short, wide connections. avoid using vias in the high-current paths. if vias are unavoidable, use many vias in parallel to reduce resistance and inductance. 2) create a power ground island (gnd1) for the step- down regulator, consisting of the input and output capacitor grounds and the gnd1 pin. connect all these together with short, wide traces or a small ground plane. similarly, create a power ground island (gnd2) for the step-up regulator, consisting of the input and output capacitor grounds and the gnd2 pin. maximizing the width of the power ground traces improves efficiency and reduces out- put voltage ripple and noise spikes. create an ana- log ground plane (gnd) consisting of the gnd pin, all the feedback-divider ground connections, the comp and del capacitor ground connections, and the device? exposed backside pad, with a large area of in-the-layer or solder-side copper with large or multiple vias to the backside pad to cool the ic. connect the gnd1, gnd2, and gnd islands by connecting the three ground pins directly to the exposed backside pad. make no other connections between these separate ground planes. rr vv vv fbn goff ref fbn 10 9 = ? ? rr v v gon fbp 78 1 = ? ? ? ? ? ? ? v nxv xnxvv n x c c v nxv xnxvv n x c c ripple pos pos supp pos d out pos pos x pos out pos ripple neg neg supp neg d out neg neg x neg out neg _ _ _ _ _ _ _ _ () = +? ? ? ? ? ? ? ? = ?+ ? ? ? ? ? ? 12 2 max8728 low-cost, multiple-output power supply for lcd monitors/tvs ______________________________________________________________________________________ 27 3) place all feedback voltage-divider resistors as close to their respective feedback pins as possible. the divider? center trace should be kept short. placing the resistors far away causes their fb traces to become antennas that can pick up switching noise. care should be taken to avoid running any feedback trace near lx1, lx2, drvp, or drvn. 4) place the inl pin and ref pin bypass capacitors as close to the device as possible. the ground connec- tion of the inl bypass capacitor should be connect- ed directly to the gnd pin with a wide trace. 5) minimize the length and maximize the width of the traces between the output capacitors and the load for best transient responses. 6) minimize the size of the lx1 and lx2 nodes while keeping them wide and short. keep the lx1 and lx2 nodes away from feedback nodes (fb1, fb2, fbp, and fbn) and analog ground. use dc traces as shield if necessary. refer to the max8728 evaluation kit for an example of proper board layout. chip information transistor count: 6752 process: bicmos max8728 low-cost, multiple-output power supply for lcd monitors/tvs 28 ______________________________________________________________________________________ package information (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation, go to www.maxim-ic.com/packages .) qfn thin.eps d2 (nd-1) x e e d c pin # 1 i.d. (ne-1) x e e/2 e 0.08 c 0.10 c a a1 a3 detail a e2/2 e2 0.10 m c a b pin # 1 i.d. b 0.35x45 d/2 d2/2 l c l c e e l c c l k l l detail b l l1 e aaaaa marking i 1 2 21-0140 package outline, 16, 20, 28, 32, 40l thin qfn, 5x5x0.8mm -drawing not to scale- l e/2 low-cost, multiple-output power supply for lcd monitors/tvs maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 ____________________ 29 2006 maxim integrated products printed usa is a registered trademark of maxim integrated products, inc. package information (continued) (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation, go to www.maxim-ic.com/packages .) common dimensions max. exposed pad variations d2 nom. min. min. e2 nom. max. ne nd pkg. codes 1. dimensioning & tolerancing conform to asme y14.5m-1994. 2. all dimensions are in millimeters. angles are in degrees. 3. n is the total number of terminals. 4. the terminal #1 identifier and terminal numbering convention shall conform to jesd 95-1 spp-012. details of terminal #1 identifier are optional, but must be located within the zone indicated. the terminal #1 identifier may be either a mold or marked feature. 5. dimension b applies to metallized terminal and is measured between 0.25 mm and 0.30 mm from terminal tip. 6. nd and ne refer to the number of terminals on each d and e side respectively. 7. depopulation is possible in a symmetrical fashion. 8. coplanarity applies to the exposed heat sink slug as well as the terminals. 9. drawing conforms to jedec mo220, except exposed pad dimension for t2855-3 and t2855-6. notes: symbol pkg. n l1 e e d b a3 a a1 k 10. warpage shall not exceed 0.10 mm. jedec 0.70 0.80 0.75 4.90 4.90 0.25 0.25 0 -- 4 whhb 4 16 0.35 0.30 5.10 5.10 5.00 0.80 bsc. 5.00 0.05 0.20 ref. 0.02 min. max. nom. 16l 5x5 l 0.30 0.50 0.40 -- - -- - whhc 20 5 5 5.00 5.00 0.30 0.55 0.65 bsc. 0.45 0.25 4.90 4.90 0.25 0.65 - - 5.10 5.10 0.35 20l 5x5 0.20 ref. 0.75 0.02 nom. 0 0.70 min. 0.05 0.80 max. -- - whhd-1 28 7 7 5.00 5.00 0.25 0.55 0.50 bsc. 0.45 0.25 4.90 4.90 0.20 0.65 - - 5.10 5.10 0.30 28l 5x5 0.20 ref. 0.75 0.02 nom. 0 0.70 min. 0.05 0.80 max. -- - whhd-2 32 8 8 5.00 5.00 0.40 0.50 bsc. 0.30 0.25 4.90 4.90 0.50 - - 5.10 5.10 32l 5x5 0.20 ref. 0.75 0.02 nom. 0 0.70 min. 0.05 0.80 max. 0.20 0.25 0.30 down bonds allowed yes 3.10 3.00 3.20 3.10 3.00 3.20 t2055-3 3.10 3.00 3.20 3.10 3.00 3.20 t2055-4 t2855-3 3.15 3.25 3.35 3.15 3.25 3.35 t2855-6 3.15 3.25 3.35 3.15 3.25 3.35 t2855-4 2.60 2.70 2.80 2.60 2.70 2.80 t2855-5 2.60 2.70 2.80 2.60 2.70 2.80 t2855-7 2.60 2.70 2.80 2.60 2.70 2.80 3.20 3.00 3.10 t3255-3 3 3.20 3.00 3.10 3.20 3.00 3.10 t3255-4 3 3.20 3.00 3.10 no no no no yes yes yes yes 3.20 3.00 t1655-3 3.10 3.00 3.10 3.20 no no 3.20 3.10 3.00 3.10 t1655n-1 3.00 3.20 3.35 3.15 t2055-5 3.25 3.15 3.25 3.35 yes 3.35 3.15 t2855n-1 3.25 3.15 3.25 3.35 no 3.35 3.15 t2855-8 3.25 3.15 3.25 3.35 yes 3.20 3.10 t3255n-1 3.00 no 3.20 3.10 3.00 l 0.40 0.40 ** ** ** ** ** ** ** ** ** ** ** ** ** ** see common dimensions table 0.15 11. marking is for package orientation reference only. i 2 2 21-0140 package outline, 16, 20, 28, 32, 40l thin qfn, 5x5x0.8mm -drawing not to scale- 12. number of leads shown are for reference only. 3.30 t4055-1 3.20 3.40 3.20 3.30 3.40 ** yes 0.05 0 0.02 0.60 0.40 0.50 10 ----- 0.30 40 10 0.40 0.50 5.10 4.90 5.00 0.25 0.35 0.45 0.40 bsc. 0.15 4.90 0.25 0.20 5.00 5.10 0.20 ref. 0.70 min. 0.75 0.80 nom. 40l 5x5 max. 13. lead centerlines to be at true position as defined by basic dimension "e", 0.05. t1655-2 ** yes 3.20 3.10 3.00 3.10 3.00 3.20 t3255-5 yes 3.00 3.10 3.00 3.20 3.20 3.10 ** exceptions |
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