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mic2130/1 high voltage synchronous buck control ic with low emi option mlf and micro leadframe are registered trademarks of amkor technology, inc. micrel inc. 2180 fortune drive san jose, ca 95131 usa tel +1 ( 408 ) 944-0800 fax + 1 (408) 474-1000 http://www.micrel.com april 2008 m9999-042108-c general description the mic2130/1 is a high voltage input pwm synchronous buck controller ic. it is a voltage mode controller with a fast hysteretic control loop (fhycl) employed during fast line and load transients. the internal gate drivers are designed to drive high current mosfets. the mic2130/1 can produce output voltages down to 0.7v with input voltage from 8v to 40v. the mic2130 family of control ics implements fix ed frequency pwm control. the active anti-shoot through drive scheme means a wide range of external mosfets may be used while maintaining optimum efficiency. the mic2131 is the fully functional version of the family and implements a new feature to minimize emi. this function is critical for systems that need to be compliant with emi standards throughout the world. the mic2130/1 is available in small size 16-pin 4mm x 4mm mlf ? package, as well as 16-pin e-tssop at a junction temperature range of C40c to +125c. data sheets and support doc umentation can be found on micrels web site at: www.micrel.com. features ? 8v to 40v input voltage range ? adjustable output voltages down to 0.7v ? low emi option mic2131 ? fixed 150khz and 400khz frequency options ? excellent line and load regulation due to fast hysteretic control loop during transients ? adaptive gate drive allows efficiencies over 95% ? programmable current limit with no sense resistor ? senses low-side mosfet current ? internal drivers allow 15a output current ? power good output allow simple sequencing ? programmable soft-start pin ? 100% increase in current limit (mic2131) ? output over-voltage protection ? programmable input uvlo ? 16-pin e-tssop and 16-pin 4mm x 4mm mlf ? ? junction temperature range of C40 c to +125 c applications ? industrial/medical dc/dc point of use power ? printer head drivers ? automotive systems ? telecom systems ? lcd/ plasma tv ? gaming machines typical application mic2130/1 high input voltage converter 80 82 84 86 88 90 92 94 96 98 efficiency v out = 3.3v @ +25c 4567 output current (a) 30v 20v 24v 12v 10v 8v 40v downloaded from: http:///
micrel, inc. mic2130/1 april 2008 2 m9999-042108-c ordering information part number frequency output voltage low emi junction temp. range package lead finish mic2130-1yml 150khz adj. no C40 to +125c 16-pin 4x4 mlf ? pb-free mic2130-4yml 400khz adj. no C40 to +125c 16-pin 4x4 mlf ? pb-free mic2130-1ytse 150khz adj. no C40 to +125c 16-pin e-tssop pb-free mic2130-4ytse 400khz adj. no C40 to +125c 16-pin e-tssop pb-free mic2131-1yml 150khz adj. yes C40 to +125c 16-pin 4x4 mlf ? pb-free mic2131-4yml 400khz adj. yes C40 to +125c 16-pin 4x4 mlf ? pb-free mic2131-1ytse 150khz adj. yes C40 to +125c 16-pin e-tssop pb-free MIC2131-4YTSE 400khz adj. yes C40 to +125c 16-pin e-tssop pb-free pin configuration hcl/agnd2 ss fb comp hsdsw pgnd lsd bst cs avdd pgood vdd vin en/uvlo agnd1 1 avdd pgood hcl/agnd2 ss fb comp agnd1 en/uvlo 16 cs bsthsd sw pgnd lsd vdd vin 1514 13 12 11 10 9 23 4 5 6 7 8 16-pin 4mm x 4mm mlf ? (ml) 16-pin e-tssop (ts) downloaded from: http:///
micrel, inc. mic2130/1 april 2008 3 m9999-042108-c pin description pin number mlf-16 pin number e-tssop-16 pin name pin function 1 3 hcl * agnd2 high current limit (output): the capacitor on this pin sets the duration of time so that t he current limit will be set to 200% of its nominal set current. agnd2 for mic2130 2 4 ss soft start (output): active at power-up, enable, and current limit recovery. 3 5 fb feedback (output): input to error amplifier. regulates to 0.7v. 4 6 comp compensation (output): pin for external compensation. 5 7 agnd1 analog ground. 6 8 en/uvlo enable (input): logic low turns the ic off. when the voltage drops below the band gap reference voltage of the ic the device turns off. this accurate threshold allows the pin to be used as an accurate under voltage lockout. 7 9 vin supply voltage (input): 8v to 40v. 8 10 vdd 5v internal linear regulator from vin. when vin is <8v, this regulator operates in drop-out mode. connect external bypass capacitor. 9 11 lsd low-side drive (output): h igh-current driver output for external low-side mosfet. 10 12 pgnd power ground: high current return for ext. low side driver. 11 13 sw switch node (output): h igh current output driver return. 12 14 hsd high-side drive (output): high current output-driver for ext. high-side mosfet. 13 15 bst boost (output): provides voltage for high-side mosfet driver. the gate drive voltage is higher than the source voltage by vdd minus a diode drop. 14 16 cs current sense (output): cu rrent-limit comparator non inverting input. the current limit is sensed across the low- side fet during the off time. current limit is set by the resistor in series with the cs pin. 15 1 avdd analog supply voltage (internal 5v ldo): connect external by pass capacitor. 16 2 pgood power good (output): high output when vout > 90% nominal. note: * indicates that is only used on mic2131. downloaded from: http:///
micrel, inc. mic2130/1 april 2008 4 m9999-042108-c absolute maximum ratings (1) supply voltage (v in ) .......................................................42v boot strapped voltage (v bst ).................................. v in + 5v logic inputs ...................................................................6.5v en input .........................................................................42v ambient storage temperature (t s ) ...........C65c to +150c esd rating (3) ............................................................... 1.5kv operating ratings (2) supply voltage (v in1, v in2 )................................ +8v to +40v output voltage ran ge.................................. 0.7v to 0.85v in junction temperature (t j ) ..................C40c t j +125c package thermal resistance e-tssop ( ja ) ................................................97.5c/w e-tssop ( jc ) ................................................29.9c/w 4x4 mlf-16 ( ja ) ............................................50.6c/w 4x4 mlf-16 ( jc ) ............................................15.8c/w electrical characteristics (4) t j = 25c; v en = v in = 24v; frequency = 150khz; v out = 3.3v, unless otherwise specified. bold values indicate C40c t j +125c. parameter condition min typ max units v in , v en/uvlo , v dd supply total supply current, pwm mode supply current v fb = 0.7v; comp = 3v (outputs switching but excluding external mosfet gate current.) 4 7.5 ma shutdown current v en = 0v; v in = 12v 180 300 a v en /uvlo turn-on threshold 1.1 1.3 v v en /uvlo turn-off threshold 0.7 1.0 v v in uvlo hysteresis 100 mv internal bias voltages (av dd ,) (i dd = 50ma) 4.6 5 5.3 v oscillator/pwm section pwm frequency mic2130/1-1 130 150 170 khz pwm frequency mic2130/1-4 360 400 440 khz maximum duty cycle mic2130/1-1 92 % maximum duty cycle mic2130/1-4 80 % minimum on-time note 5 50 ns regulation feedback voltage reference (2%) -40 c to +125 c 686 700 714 mv feedback bias current 300 1000 na output voltage line regulation v in = 8v C 40v 0.03 %/v output voltage load regulation 1a i out 10a 0.5 % error amplifier (each channel) dc gain 70 db output impedance 2 m ? transconductance 1.2 1.6 2.5 ms output over voltage protection v fb threshold (latches lsd high) 110 115 120 %nom delay blanking time 1 s soft start/hcl internal soft start source current 1 2.75 5 a hcl (mic2131) pin voltage high i limit set to 200% when hcl charges to2.4v 2.4 v downloaded from: http:///
micrel, inc. mic2130/1 april 2008 5 m9999-042108-c parameter condition min typ max units hcl low i limit set to normal when hcl is low 0.5 v hcl pin charge up current 2.9 a hcl pin charge down current 12 a low/emi frequency dither range of center freq 12 % current sense cs over current trip point program current 170 200 230 a temperature coefficient +2300 ppm/ c cs comparator sense threshold (senses drop across low-side fet) C5 0 +5 mv power good v fb threshold 86 90 93 %nom pgood voltage low v dd = 5.0v; v fb = 0v; i pgood = 1ma 0.1 0.5 v gate drivers rise/fall time into 3000pf source sink 23 16 ns ns output driver resistance hsd: source; v dd = 5v hsd: sink; v dd = 5v lsd: source; v dd = 5v lsd: sink; v dd = 5v 2 1.47 2.2 2.1 ? ? ? ? driver non-overlap time (adaptive) note 4 40 60 ns t j increasing 155 c thermal shutdown threshold t j decreasing 142 c notes: 1. exceeding the absolute maximum rating may damage the device. 2. electrical specifications do not apply when operating the device outside of its oper ating ratings. the maximum allowable p ower dissipation is a function of the maximum junction temperature, t j(max) , the junction-to-ambient thermal resistance, ja , and the ambient temperature, t a . the maximum allowable power dissipation will result in exce ssive die temperature, and the regulator will go into thermal shutd own. 3. devices are esd sensitive. handling precautions recommended. 4. guaranteed by design. 5. minimum on-time before automatic cycle skipping begins. downloaded from: http:///
micrel, inc. mic2130/1 april 2008 6 m9999-042108-c typical characteristics 3.4 5.4 3.6 3.8 4.0 4.2 4.8 5.0 5.2 supply current vs. temperature 20 40 60 80 temperature (c) 4.4 4.6 24v 8v 12v 40v 42v 1.10 1.12 1.14 1.16 1.18 1.20 1.22 enable voltage on vs. temperature 20 40 60 80 temperature (c) 0.94 0.96 0.98 1.00 1.02 1.04 1.06 1.08 1.10 1.12 1.14 enable voltage off vs. temperature 20 40 60 80 temperature (c) 0 100 200 300 400 500 600 700 800 shutdown current vs. temperature 20 40 60 80 temperature (c) 24v 8v 12v 40v 42v 185 190 195 200 205 210 cs current vs. temperature 20 40 60 80 temperature (c) 24v 8v 12v 40v 42v 0.692 0.694 0.696 0.698 0.700 0.702 v fb vs. temperature 20 40 60 80 temperature (c) 24v 8v 12v 40v 42v 4.82 4.84 4.86 4.88 4.90 4.92 4.94 4.96 4.98 av dd vs. temperature 20 40 60 80 temperature (c) 24v 8v 12v 40v 42v 0 0.05 0.10 0.15 0.20 0.25 0.30 p good low @ i pg = 1ma vs. temperature 20 40 60 80 temperature (c) 0.605 0.606 0.607 0.608 0.609 0.610 0.611 0.612 p good threshold vs. temperature 20 40 60 80 temperature (c) 80 82 84 86 88 90 92 94 96 98 efficiency v out = 3.3v @ +25c 4567 output current (a) 30v 20v 24v 12v 10v 8v 40v 110 115 120 125 130 135 140 145 150 155 160 165 170 dithered (spread) frequency vs. temperature 20 40 60 80 temperature (c) 125 130 135 140 145 150 155 switch frequency vs. temperature 20 40 60 80 temperature (c) v comp >2.1v v comp =1.5v v comp =2.1v v comp =1.1v downloaded from: http:///
micrel, inc. mic2130/1 april 2008 7 m9999-042108-c typical characteristics (continued) 3.312 3.314 3.316 3.318 3.32 3.322 3.324 load regulation @ +85c 4567 output current (a) 30v 20v 24v 12v 10v 8v 40v 3.354 3.356 3.358 3.36 3.362 3.364 5 10152025303540 input voltage (v) line regulation @ 2 amp load downloaded from: http:///
micrel, inc. mic2130/1 april 2008 8 m9999-042108-c functional diagram figure 1. mic2130/1 block diagram functional description the mic2130/31 is a voltage mode synchronous buck controller built for optimum speed and efficiency. it is designed for wide input voltage range and for high output power buck converters. figure 1 shows the block diagram. the control loop has two stages of regulation. during steady-state to medium out put disturbances, the loop operates in fixed frequency, pwm mode while (gm loop), during a large output voltage disturbance (~6% nominal), the loop becomes hysteretic; meaning that for a short period, the switching mosfets are switched on and off continuously until the output voltage returns to its nominal level. this maximizes transient response for large load steps, while nominally operating in fixed frequency pwm mode. voltage mode control is used to allow for maximum flexib ility and maintains good transient regulation. the operating input voltage range is 8v to 40v and output can be set from 0.7v up to 0.85*v in . start-up surges are prevented using built in soft start circuitry as well as resistor-less (lsd rds on is used to sense load current) current sensing for overload protection. other protection features include uvlo, over voltage latch off protection, power good signal. downloaded from: http:///
micrel, inc. mic2130/1 april 2008 9 m9999-042108-c theory of operation a voltage divider monitors the output voltage of the converter then sensed at the inverting input of the error amplifier. the non-inverting input of the error amplifier is connected to the internal 0.7v reference and the two inputs are compared to produce an analog error voltage. this error voltage is then fed into the non-inverting input of the pwm comparator and compared to the voltage ramp (1.1v to 2.1v) to create the pwm pulses. the pwm pulses propagate through to the mosfet drivers which drive the external mosfets to create the power switching waveform at the set d (duty cycle). this is then filtered by a power inductor and low esr capacitor to produce the output voltage where v out d*v in . as an example, due to a load increase or an input voltage drop, the output voltage will instantaneously drop. this will cause the error voltage to rise, resulting in wider pulses at the output of the pwm comparator. the higher duty cycle power switchi ng waveform will cause an associated rise in output vo ltage and will continue to rise until the feedback voltage is equal to the reference and the loop is again in equilibri um. as with any control system, it is necessary to compensate this feedback loop (by selecting the r and c values at the comp pin) in order to keep the system stable. one of the tradeoffs for stability is reduced transient regulation performance. however, the mic2130/31 has an additional feature to correct this problem. the mic2130/31 family features a fast hysteretic control loop (fhycl) which bypasses the gm amp and the feedback compensation network during fast line and load transients. the fast hysteretic control loop (fhycl) operates during large transients to provide excellent line and load regulation. hysteretic mode is invoked when the output voltage is detected to be 6% of its regulated value. if the input voltage step or output load step is large enough to cause a 6% deviation in v out , then the additional hysteretic control loop functions to return the output voltage to its nominal set point in the fastest time possible. this is limited only by the time constant of the power inductor and output capacitor (an order of magnitude faster than the gm loop). this scheme is not used during normal operation because it creates switching waveforms whose frequency is dependant on v in , passive component values and load current. due to its large noise spectrum it is only used during surges to keep switching noise at a known, fixed frequency. figure 2. hysteric block diagram figure 3. hysteric waveforms soft start figure 4. soft start circuit downloaded from: http:///
micrel, inc. mic2130/1 april 2008 10 m9999-042108-c at startup, the soft-start mosfet (ss fet ) is released and c ss starts to charge at the rate dv ss /dtC2a/c ss . the pnp transistors emitter (comp) starts to track v ss (plus a junction voltage 0.65). when (comp) reaches the lower end of the pwm ramp voltage at 1.10v, switching pulses will begin to drive the power mosfets. this voltage rise continues on the comp pin until the control loop reaches the regulation point. during this soft start period, the gate drive pulses to the mosfet will start at the minimum pulse width and increase up to the duty cycle d required for regul ation. the comp voltage can be anywhere from 1.1v to 2.1v which corresponds to a duty cycle d of 0-85%. v ss will however, continue to rise as the pnp base-emitter junction becomes reverse biased. the ss pin is allowed to rise to 2.5v (four diode drops) max to allow fast response to fault conditions. during large over current or short circuit conditions, i.e., where current limit is detected and v out is <60% of nominal, the ss fet is momentarily switched on. this discharges c ss to ~150mv at which point, it re-starts the soft start cycle once again. during soft start, hysteretic comparators are disabled. duty cycle d can be written in terms of v comp d = (0.85) x v comp C (0.935) or v comp = (d + 0.935)/0.85 example: v in = 12v; v out = 3.3v; d = v out /v in = 0.275 v comp = 1.424v; i.e. the steady-s tate dc value of v comp when d = 0.275 t1 is the time for v comp to charge up to 1.1v, therefore, v ss is one diode drop below v comp . t2 is the time for v comp to charge up to (d + 0.935)/ 0.85 + 1.1v. soft start time = t1 + t2 where t1 = (1.1-vdiode) x c ss /2a; the time until output pulsing starts at minimum duty. and, t2 = (1/0.85) x d x c ss / 2a; the time until output pulsing increases to d. the compensation capacitors at the comp pin (c comp = cc1+cc2 in figure 4) will also need to charge up to v comp . this charging time starts as soon as mosfet (ss fet ) is released. depending upon the size of the c comp , the charging time could be greater than t1+t2. c comp could be used for the soft start cap by leaving ss pin open. tc comp = (1/0.85) x d x c comp /5a: the time until output pulsing increases to d. protection there exits four different ty pes of output protection. 1. output hard short over current 2. output soft short over current 3. output under voltage 4. output over voltage current limit the mic2130/31 uses the rds on of the low-side mosfet to sense overcurrent conditions. the lower mosfet is used because it displays much lower parasitic oscillations during switching then the upper mosfet. using the lo w-side mosfet rds on as a current sense is an excellent method for circuit protection. this method w ill avoid adding cost, board space and power losses taken by discrete current sense resistors. hard short generally, the mic2130/31 current limit circuit acts to provide a fixed maximum output current until the resistance of the load is so low that the voltage across it is no longer within regulation limits. at this point (60% of nominal output voltage), t he part employs hiccup mode. during hiccup mode, the output pulses stop and the soft start cap is discharged and soft start mode begins. after the soft start time, if the out put voltage is still 60% low, then the process repeats agai n and continues until the short is removed. hard short current mode is initiated to protect down stream loads from excessive current and also reduces overall power dissipation in the pwm converter components during a fault. soft short before hard short mode (also called hiccup mode) occurs soft short current lim iting is provided to prevent system shutdown or disturbance if the overload is only marginal. when the load current exceeds the current limit by only a few ma for a sh ort time (milliseconds) then the hard short mode is not desired. instead, the soft short loop is used. when t he current limit comparator senses an over current it then starts to discharge the ss cap with a 40a current source. the current limit comparator gets reset every cycle so if the short still exist during the next cycle t hen the ss cap will continue to get discharged with the 40a current source. the comp pin follows the ss pin (figure 4) and the gm control loop will lower the out put voltage accordingly for as long as the short exists. so, instead of shutting down the output as in a hard shor t, the output is gently and slightly reduced until the over current condition discontinues. if however the short increase to the point of lowering the output to 60%, then hard short will result. the fast hysteretic control l oop (fhycl) is initiated by a 6% drop in output voltage and it is not desired during an over current condition theref ore, the fhycl feature will downloaded from: http:///
micrel, inc. mic2130/1 april 2008 11 m9999-042108-c be disabled during an over current condition. under voltage a 6% comparator monitors the output voltage and will initiate the fast hysteretic control loop (fhycl) to regulate the output. a comparator monitors the output voltage and sets pgood true when the output reaches 90% of the regulated output. over voltage if the voltage at the fb pin is detected to be 15% higher than nominal for >2s, then the controller is stopped from switching immediately and latched off. switching can be re-started by taking en below the channels enable threshold and re-enabling or re-cycling power to the ic. figure 5. during the normal operation of a synchronous buck regulator, as the lower mosfet is switched on, its drain voltage will become negative wi th respect to ground as the inductor current continue s to flow from source-to- drain. this negative voltage is proportional to output load current, inductor ripple current and mosfet rds on . figure 6. the larger the inductor current, the more negative v ds becomes. this is utilized for the detection of over current by passing a known fixed current source (200a) through a resistor r cs which sets up an offset voltage such that when 200a x r cs = i drain x rds on the mic2130/31s over current trigger is set. this disables the next high side gate drive pulse. after missing the high side pulse, the over current (oc) trigger is reset. if on the next low side drive cycl e, the current is still too high i.e., v cs is 0v, another high side pulse is missed and so on. thus reducing the overall energy transferred to the output and v out starts to fall. as this successive missing of pulses results in an effectively lower switching frequency, power inductor ripple currents can get very high if left unlimited. the mi c2130/31 therefore limits duty cycle during current limit to prevent currents building up in the power inductor and output capacitors. current limit setting the simple method r cs = i out x rds on(max) /200a. accurate method for designs where ripple current is significant when compared to i out, or for low duty cycle operation, calculating the current setting resistor r cs should take into account that one is sensing the peak inductor current and that there is a blanking delay of approximately 100ns. figure 7. 2 ripple out pk i i i + = l f d v i switch out ripple ? ? ? = ) 1( l t v i i dly out pk set ? ? = (min) (max) cs on set cs i rds i r ? = d = duty cycle f switch = switching frequency downloaded from: http:///
micrel, inc. mic2130/1 april 2008 12 m9999-042108-c l = power inductor value t dly = current limit blanking time ~ 100ns i cs(min) = 180a example: consider a 12v to 3.3v @ 5a converter with 7.3h power inductor and 93% efficiency at a 5a load and an lsd fet of rds on of 10m ? (typical values). efficiency v v d in out ? = a h khz i ripple 1.2 3.7 150 ) 306 .0 1(3.3 = ? ? ? = a i pk 05.6 2 1.2 5 = + = a h ns i set 00.6 3.7 100 3.3 05.6 = ? ? + ? = ? ? = 333 180 10 00.6 a m r cs (332 std. value) using the simple method here would result in a current limit point lower than desired. this equation sets the minimum current limit point of the converter, but maximum w ill depend upon the actual inductor value and rds on of the mosfet under current limit conditions. this could be in the region of 50% higher and should be considered to ensure that all the power components are within their thermal limits unless thermal protection is implemented separately. hcl (mic2131 only) the high current limit (hcl) is a function of the mic2131 only. it allows for twice the out put load current (for a time t determined by the hcl cap) before the current limit comparator trips. during the time t, the current sense current source (200a nominal) is increased to 400a. t = chcl * 2/13 = chcl * 153.85 *1e3 where chcl is the cap at the hcl pin frequency dithering the mic2131 has an additi onal useful feature. the switching frequency is dithered 12% in order to spread the frequency spectrum over a wider range to lower the emi noise peaks generated by the switching components. a pseudo random generator is used to generate the dithering which further reduces the emi noise peaks. power good output the power good out put (pg) will go high only when output is above 90% of the nom inal set output voltage. vdd regulator the internal regulator provides a regulated 5v for supplying the analog circuit power (av dd ). v dd also powers the mosfet drivers. v dd is designed to operate at input voltages down to 8v. the av dd supply should be connected to v dd through an rc filter to provide decoupling of the switching noise generated by the mosfet drivers taking large current pulses from the v dd regulator. gate drivers the mic2130/31 is designed to drive both high side and low side n-channel mosfets to enable high switching speeds with the lowest possible losses. the high side mosfet gate driver is supplied by a bootstrap capacitor cbst connected at the sw pin and the bst pin. a high speed diode (a schottky diode is recommended) between the v dd pin and bst pin is required as shown in figure 8. this provides the high side mosfet with a constant vgs drive voltage equal to v dd - v diode . figure 8. when hsd goes high, this turns on the high side mosfet and the sw node rises sharply. this is coupled through the bootstrap capacitor cbst and diode dbst becomes reverse biased. the mosfet gate is held at v dd -v diode above the source for as long as cbst remains charged. this bias current of the high side driver is <10ma so a 0.1f to1 f is sufficient to hold the gate voltage with mi nimal droop for the power stroke (high side switching) cycle, i.e. ? bst = 10ma x 6s / 0.1f = 567mv. when the low side driver turns on every switching cycle, any lost charge from cbst is replaced via dbst as it becomes forward biased. therefore minimum bst voltage is v dd C 0.5v. the low side driver is supplied directly from v dd at nominal 5v. downloaded from: http:///
micrel, inc. mic2130/1 april 2008 13 m9999-042108-c adaptive gate drive there is a period when both driver outputs are held off (dead time) to prevent shoot through current flowing. shoot through current flows if both mosfets are on momentarily as the same time and reduces efficiency and can destroy the fets. this dead time must be kept to a minimum to reduce losses in the free wheeling diode which could either be an external schottky diode placed across the lower mosfet or the internal schottky diode implemented in some mosfets. it is not recommended, for high current designs, to rely on the intrinsic body diode of the power mosfet. these typically have large forward voltage drops and a slow reverse recovery characteristic which will add significant losses to the regulator. dependant upon the mosfets used, the dead time could be required to be 150ns or 20ns. the mic2130/ 31 solves this variability issue by using an adaptive gate-drive scheme. when the high side driver is turned off, the inductor forces the voltage at the switching node (low side mosfet drain) towards ground to keep current flowing. when the sw pin is detected to have reached 1v, the top mosfet can be assumed to be off and the low side driver output is immediately turned on. there is also a short delay between the low side drive turning off and the high side driver turning on. this is fixed at ~80ns to allow for large gate charge mosfets to be used. downloaded from: http:///
micrel, inc. mic2130/1 april 2008 14 m9999-042108-c application information passive component selection guide transition losses in the power mosfets are not defined by inductor value. however, the inductor value is responsible for the ripple current which causes some of the resistive losses. these losses are proportional to i ripple 2 . minimizing inductor ripple current therefore reduces resistive losses and can be achieved by choosing a larger value inductor. this will generally improve efficiency by reducing the rms current flowing in all of the power components. the actual value of inductance is really defined by space limitations, rms rating (i rms ) and saturation current (i sat ) of available inductors. if we look at the newer flat wire inductors, these have higher saturation current ratings than the rms current rating for lower values and as inductance value increases, these figures get closer in value. this mirrors what happens in the converter with i sat analogous to the maximum peak switch current and i rms analogous to output current. as inductance increases, so i switchpk tends towards i out . this is a characteristic that makes these types of inductor optimal for use in high power buck converters such as mic2130/31. to determine the i sat and i rms rating of the inductor, we should start with a nominal value of ripple current. this should typically be no more than i out(max) /2 to minimize mosfet losses due to ripple current mentioned earlier. therefore: ? ? ? ? ? ? ? ? ? ? ? ? efficiency v v f i v l in o switch o o min 1 2 ~ il rms > 1.04 x i out(max) il sat > 1.25 x i out(max) any value chosen above l min will ensure these ratings are not exceeded. in considering the actual value to choose, we need to look at the effect of rippl e on the other components in the circuit. the ch osen inductor value will have a ripple current of: () l v f d i out switch ripple ? ? 1 ~ this value should ideally be ke pt to a minimum, within the cost and size constraints of the design, to reduce unnecessary heat dissipation. output capacitor selection the output capacitor (c out ) will have the full inductor ripple current il rms flowing through it. this creates the output switching noise which consists of two main components: 4 43 4 42 1 4 43 4 42 1 noise capacitor out on ripple noise esr ripple pk outpk c t i esr i v ? ? + ? ? 2 if therefore, the need is for low output voltage noise (e.g., in low output voltage converters), v out ripple can be directly reduced by increasi ng inductor value, output capacitor value or reducing esr. for tantalum capacitors, esr is typically >40m ? which usually makes loop stabilizat ion easier by utilizing a pole-zero (type ii) compensator. due to the many advantages of multi-layer ceramic capacitors, among them, cost, size, ripple rating and esr, it can be useful to use these in many cases. however, one disadvantage is the cv product. this is lower than tantalum. a mixtur e of one tantalum and one ceramic can be a good compromise which can still utilize the simple type ii compensator. with ceramic output capacitors only, a double-pole, double-zero (type iii) compensator is r equired to ensure system stability. loop compensation is described in more detail later in the data sheet. ensure the rms ripple current rating of the capacitor is above i ripple ? 0.6 to improve reliability. input capacitor selection the input filter needs to supply the load current when the high-fet is on and should to lim it the ripple to the desire value. the c in ripple rating for a converter is typically i out /2 under worst case duty cycle conditions of 50%. irms cin = ( ) d d i out ? 1 where d = v out /(v in xeff) it is however, also important to closely decouple the power mosfets with 2 x 10f ceramic capacitors to reduce ringing and prevent noise related issues from causing problems in the lay out of the regulator. the ripple rating of c in may therefore be satisfied by these decoupling capacitors as they allow the use of perhaps one more ceramic or tantalum input capacitor at the input voltage node to decouple input noise and localize high di/dt signals to the regulator input. power mosfet selection the mic2130/31 drives n-channel mosfets in both the high-side and low-side positions. this is because the switching speed for a given rds on in the n-channel device is superior to the p-channel device. there are different criteria for choosing the high- and low-side mosfets and these differences are more significant at lower duty cycles such as 12v to 1.8v conversion. in such an application, the high-side mosfet is required to switch as quickly as possible to minimize transition losses (power dissipated during rise downloaded from: http:///
micrel, inc. mic2130/1 april 2008 15 m9999-042108-c and fall times). whereas the low-side mosfet can switch slower, but must handle larger rms currents. when duty cycle approaches 50%, the current carrying capability of the upper mosfet starts to become critical also and can sometimes require external high current drivers to achieve the nec essary switching speeds. mosfet loss = static loss + transition loss static loss (ps) = i fetrms 2 x rds on transition loss (pt) = i out x (tr+tf) x v dsoff x f switch /2 tr + tf = rise time + fall time due to the worst case driver currents of t he mic2130/31, the value of tr + tf simplifies to: tr + tf (ns) = ? qg (nc) ? qg can be found in the mosfet characteristic curves v dsoff = voltage across mosfet when it is off i fetrms = 3 ) ( 2 2 y y x x i i i i d + ? + ? i x = i out C i ripple /2 i y = i out + i ripple /2 d_on = t on x f switch high-side fet on time d_off = t on x f switch low-side fet on time d_on = d = v o /(v in x eff) since it changes depending on which mosfet we are calculating losses for. high-side fet t on = d_on/f switch the lower mosfet is not on for the whole time that the upper mosfet is off due to the fixed 80ns high side driver delay. therefore, there is an 80ns term subtracted from the lower fet on time equation. low-side fet t on = (1-d_on)/f switch C 80ns there are many mosfet packages available which have varying values of thermal resistance and can therefore dissipate more pow er if there is sufficient airflow or heat sink externally to remove the heat. however, for this exercise we can assume a maximum dissipation of 1.2w per mosfet package. this can be altered if the final design has higher allowable package dissipation. look at lower mosfet first: pdis_max = 1.2 w = ps + pt for the low side fet, pt is small because vds off is clamped to the forward vo ltage drop of the schottky diode. therefore: rds on(max) ~ 1.2/i fetrms 2 example: for 12v to 1.8v @ 10a rds on(max) < 14m ? it is important to remember to use the rds on(max) figure for the mosfet at the maximum temperature to help prevent thermal runaway (as the temperature increases, the rds on increases). ? qg (max) should be limited so that the low side mosfet is off within the fixed 80ns delay before the high side driver turns on. high side mosfet: for the high side fet, the losses should ideally be evenly spread between transit ion and static losses. use the c of the v in range to balance the losses. pt = pdis_max/2 = 0.6 = i out x ? qg x v inmid x f switch /2 ? qg (max) < 0.6 x 2 / (i out x v inmid x f switch ) rdson is calculated similarly for the high side mosfet: rds on(max) ~ 0.6 / i fetrms 2 using previous example: ? qg (max) < 20nc rds on(max) < 35m ? note that these are maximum values based upon thermal limits and are not targeted at the highest efficiency. selection of lower values is recommended to achieve higher efficiency designs. limits to watch out for: qg total < 1500 nc/v in total of both high side and low side mosfet qg value at vgs = 5v for both channels. example: @ v in(max) = 13.2v, qg total < 1500/13.2 = 114nc ? qg low < 120nc maximum turn on gate charge for the low side mosfet to ensure proper turn off before high side mosfet is switched on. downloaded from: http:///
micrel, inc. mic2130/1 april 2008 16 m9999-042108-c control loop stability and compensation figure 10 shows the simplifi ed system schematic. the internal transconductance error amplifier is used for compensating the voltage feedback loop by placing a capacitor (c1) in series with a resistor (r1) and another capacitor c2 in parallel from the comp pin-to-ground. (note: ceramic output caps may require type iii compensation). r1 c1 c2 gm v ref v fb v comp d (s) v ramp r hs r ls v in v out r esr r load c pwm comparator l v sw figure 10 simplified system schematic in order to have a stable system when the gain of t(s) = 1 ? (0db) the phase has to be greater (less negative) than -180. the amount the ph ase is greater than -180 is called the phase margin, typically 30 to 60 and is a key parameter predicting the stability of the system and how much overshoot a nd undershoot the system exhibits during transients. the open loop transfer function is: t(s) = g ea (s) x g pwmcomp (s) x g pwrs (s) x g flt (s) x h fb (s) and the phase is: fb flt pwrs pwmcomp ea t (s) t(s) + + + + = = v ref v fb v out g ea (s) = g m z comp g pwmcomp g pwrs g flt h(s) v e v comp d (s) v sw t(s) figure 11 system block diagram in the system block diagram in figure 11 g ea (s) = g m x z comp gain of the error amp where g m =1.5ms and ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? + = 2 1 comp sc 1 sc 1 r1 z 0.85 1.1 2.1 0.85 ? v d ? v ? d (s) g ramp max comp pwmcomp = ? = = = gain of the pwm comparator d * d v d * ? d(s) v ? d(s) ? v (s) g max out out sw pwrs = = = gain of the power stage 2 0 2 0 out esr flt s q s 1 c sr 1 (s) g + + + = gain of the filter where: out load lost stored l/c r e e q = = (parallel loaded) and out 0 lc 1 = out esr esr c r 2 1 f = out ref hs ls ls v v r r r h(s) = + = gain of the feedback network for simplicity, combine the pwm comparator gain and the power stage gain and call it the modulator gain. d * ? v v d * d v * ? v d (s) g* (s) g (s) g ramp out max out ramp max pwrsw pwmcomp mod = = = therefore; t(s) = g ea (s) x g mod (s) x g flt (s) x h fb (s) and fb flt mod ea t (s) + + + = the phase of the open loop is the phase of all the blocks in the loop added together. the phase of t(s) is fb flt pwrs pwmcomp ea t (s) + + + + = fb mod ea t (s) + + = where pole1 zero1 pole0 ea ? + ? = pole0 = phase lag due to the pole at the origin zero1 =phase lead due to zero1 pole1 = phase lag due to pole1 = 0 pwmcomp , = 0 pwrs = 0 fb therefore; = 0 mod the phase of the filter includes the complex poles of lc and the zero caused by the esr of the c out . the filter has 2 poles at 0 f and a zero at esr f ? = 180 flt at 0 f and +90 at esr f downloaded from: http:///
micrel, inc. mic2130/1 april 2008 17 m9999-042108-c example: v in = 24v; v out = 3.3v; i out = 10a; l = 7.3h; c out = 660f; resr = 40m ? ; fsw = 150khz the gain and phase of the m odulator and filter is: g mod (s) x g flt (s) this is the gain comp out v v (s) in figure 12 a computer generated plot of g mod (s) x g flt (s) is shown in figure 12. figure 12. modulator transfer function there is a -180 phase change near f 0. at frequencies greater then f 0 the phase increases towards -90 due to the zero of f esr . the phase effects of poles and zeros start a decade below and finish a decade above the frequency of a pole or zero. t herefore, at the frequency of a pole or zero the phase effect is only half of the final value. at the complex pole 2. 3khz the phase is -90 and would be -180 at 23khz if not for the +90 phase lead of the zero at around 6khz due to the esr of the filter capacitors. (actually, the phase gain plots reach their final values asymptotically). by inspecting figure 12 the dc and low frequency gain of g mod = 20log(0.85 x 24) = 26.2db; f 0 = 2.3khz; f esr = 6khz and q is 13.6db. the peak gain equals the low freq gain plus the q = 26.2 + 13.6 = 39.9db. it is desired that t(s) (the open loop transfer function) have a cross over frequency (fco) of 1/10 the switching frequency at 15khz. it is require that fco) t(j 2 (the phase of t(s) at fco), to be greater than -180 by at least the phase margin. by inspecting the gain plot of g mod (s) at 15khz, g mod (s) has a gain of about 3.9db. therefore, to make t(j2 fco) = 1 0db; t(s) = g ea (s) x g mod (s) x h fb (s) = 1 at fco h fb = vref/vout = 0.7/3.3 = 0.212 -13.5db |g ea | db = |t| db - |g mod | db -|h fb | db = 0-3.9db C (-13.5db) = 9.6db T >3.02 the error amp needs 9db of gain at fco. therefore g m x z comp = 3.02 at 15khz. the location of the error amps zero and poles are selected in order to achieve the desired phase margin of t(s). for the maximum phase boost at the cross over frequency (fco), place the first zero1 of the ea at fco/10 since the effect of its phase boost will be at the maximum at fco. likewise, place the pole of the ea at least 10 x fco so the effects of its phase lag will be at a minimum at fco. therefore, use r1 = 2k; c1 = 0.068f; c2 = 470pf. g m error amplifier usually, it is undesirable to have high error amplifier gain at high frequencies otherw ise high frequency noise spikes at large amplitude would be present at the output. hence, gain should be permitted to fall off at high frequencies. at low frequency, it is desired to have high open-loop gain to attenuate the power line ripple. thus, the error amplifier gain should be allowed to increase rapidly at low frequencies. the transfer function for the internal gm error amplifier with r1, c1, and c2 at the comp pin is given by the following equation: ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? + ? + ? ?? + ? = 2 1 2 1 1 1 )2 1( 1 1 1 )( c c s c c r c c s c s r g s g m ea the above equation can be simplified by assuming c2 micrel, inc. mic2130/1 april 2008 18 m9999-042108-c figure 14 shows the gain and phase curves for the above transfer function with r1 = 2k, c1 = 0.068 f, c2 = 470pf, and gm = 0.0015 ? C1. it can be seen that at 15khz, the error amplifier ex hibits approximately 9.6db of gain and 170 of phase. figure 13 shows the open loop transfer function t(s) with these compensation values. it has a cross over frequency of 15khz and phase margin of 60 ? . figure 13. error amp gain and phase figure 14. the open loop t(s) gain and phase downloaded from: http:///
micrel, inc. mic2130/1 april 2008 19 m9999-042108-c package information 16-pin e-tssop (ts) 16-pin 4mm x 4mm mlf ? (ml) downloaded from: http:///
micrel, inc. mic2130/1 april 2008 20 m9999-042108-c micrel, inc. 2180 fortune drive san jose, ca 95131 usa tel +1 (408) 944-0800 fax +1 (408) 474-1000 web http://www.micrel.com the information furnished by micrel in this data sheet is belie ved to be accurate and reliable. however, no responsibility is a ssumed by micrel for its use. micrel reserves the right to change circuitry and specifications at any time without notification to the customer. micrel products are not designed or authori zed for use as components in life support appliances, devices or systems where malfu nction of a product can reasonably be expected to result in personal injury. life suppo rt devices or systems are devices or systems that (a) are in tended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significan t injury to the user. a purchasers use or sale of micrel produc ts for use in life support app liances, devices or systems is a purchasers own risk and purchaser agrees to fully indemnify micrel for any damages resulting from such use or sale. ? 2007 micrel, incorporated. downloaded from: http:///

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