microsemi linfinity microelectronics division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 1 copyright ? 2000 rev. 1.1,2003-09-04 www. microsemi . com lx1681/1682 v olta g e-mode pwm controllers p roduc tion a microsemi company l infinity description the lx1681/1682 are monolithic, pulse-width modulator controller ics. they are designed to implement a flexible, low cost buck (step-down) regulator supply with minimal external components. the lx1681 is a non-synchronous controller; the lx1682 has a synchronous driver for higher efficiency. the output voltage is adjustable by means of a resistor divider to set the voltage between 1.25v and 4.5v. short-circuit current limiting can be implemented without expensive current sense resistors. current is sensed using the voltage drop across the r ds(on) of the mosfet ? sensing is delayed for 1s to eliminate mosfet ringing errors. hiccup-mode fault protection reduces average power to th e power elements during short-circuit conditions. switching frequency is fixed at 200khz for optimal cost and space. under-voltage lockout and soft-start for optimal start-up performance. pulling the soft-start pin to ground can disable the lx1681/82. small 8-pin soic packaging reduces board space. optimized for 5v-to-3.3v or 5v-to-2.5v conversion, the lx1681/82 can also be used for converting 12v to 5v, 3.3v or other voltages with high efficiency, eliminating the need for bulky heat sinks. important: for the most current data, consult microsemi ?s website: http://www.microsemi.com key features ? fixed 200khz switching frequency ? constant frequency voltage-mode control requires no external compensation ? hiccup-mode over-current protection ? high efficiency ? output voltage set by resistor divider ? under-voltage lockout ? soft-start and enable ? synchronous rectification (lx1682) ? non-synchronous rectification (lx1681) ? small, 8-pin surface mount package applications ? 5v to 3.3v or less buck regulators ? fpga supplies ? microprocessor chipset supplies (e.g. camino, whitney, etc.) ? rambus ? rimm? supplies ? hard disk drives ? computer add-on cards product highlight lx1681 0.1f q 1 d 2 l 1 c 2 r 1 r 2 r set c 1 c 3 v fb c ss n.c. gnd tdrv v c1 cs v c2 v boost 12v v in 5v v out ss lx1681 non-synchronous controller 5h 1500f x3 irl3103s mbr2545 1500f x3 1f lx1682 0.1f q 1 l 1 c 2 r 1 r 2 r set c 1 v fb c ss bdrv gnd tdrv v c1 cs v c2 v in 5v v out ss q 2 c 3 v boost 12v lx1682 synchronous controller 1500f x3 5h irl3103s irl3103s 1500f x3 1f package order info t a ( c) o utput dm plastic soic 8-pin 0 to 70 non-synchronous lx1681cdm synchronous lx1682cdm note: available in tape & reel. append the letter ?t? to the part number. (i.e. lx1681cdmt) l l x x 1 1 6 6 8 8 1 1 / / 1 1 6 6 8 8 2 2
microsemi linfinity microelectronics division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 2 copyright ? 2000 rev. 1.1,2003-09-04 www. microsemi . com lx1681/1682 v olta g e-mode pwm controllers p roduc tion a microsemi company l infinity absolute maximum ratings (note 1) thermal data dm p ackage thermal resistance - junction to ambient , ja 165 c/w junction temperature calculation: t j = t a + (p d x ja ). the ja numbers are guidelines for the therma l performance of the device/pc-board system. all of the above assume no ambient airflow. package pin out 1 2 4 3 5 6 8 7 v fb ss n.c. gnd v cc cs v c1 tdrv lx1681 dm p ackage (top view) 1 2 4 3 5 6 8 7 v fb ss gnd bdrv v cc cs v c1 tdrv lx1682 dm p ackage (top view) functional pin description p in n ame d escription v fb voltage feedback. a 1.25v reference is connected to a resistor divider to set desired output voltage. ss soft-start and hiccup capacitor pin. during start up th e voltage of this pin controls the output voltage. an internal 20k ? resistor and the external capacitor set the time constant for soft-startup. soft-start does not begin until the supply voltage exceeds the uvlo th reshold. when over-current occurs, this capacitor is used for timing hiccup. the pwm can be disabled by pulling the ss pin below 0.3v gnd ground for ic. tdrv gate drive for upper mosfet. bdrv gate drive for lower mosfet. v c1 separate supply for mosfet gate drive. connect to 12v. cs over-current set. connect resistor between cs pin and th e source of the upper mosfet to set current-limit point. v cc ic supply voltage (nominal 5v) and high side drain sense voltage. supply voltage (v c1 ) ....................................................................................................18v supply voltage (v cc ) ......................................................................................................7v output drive peak curre nt source ( 500ns) .................................................................. 1.0a output drive peak curre nt sink ( 500ns) ......................................................................1.0a input voltage (ss/enab le pin)........................................................................ -0.3 to 6v operating junction temperatur e ............................................................................... 150c storage temperature .................................................................................-65c to +150c lead temperature (solde ring 10 sec onds)................................................................ 300c note: exceeding these ratings could cause damage to th e device. all voltages are with respect to ground. currents are positive into, ne gative out of specified terminal. p p a a c c k k a a g g e e d d a a t t a a
microsemi linfinity microelectronics division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 3 copyright ? 2000 rev. 1.1,2003-09-04 www. microsemi . com lx1681/1682 v olta g e-mode pwm controllers p roduc tion a microsemi company l infinity electrical characteristics unless otherwise specified, the following specificati ons apply over the operat ing ambient temperature 0 c t a 70 c except where otherwise noted. test conditions: v cc =5v, v c1 =12v, t=25c lx1681/1682 parameter symbol test conditions min typ max units ` reference reference voltage v fb v out =v fb , t a =25c 1.237 1.25 1.262 v v out =v fb , 0c < t a < 70c 1.231 1.269 v ` oscillator frequency f osc 170 190 230 khz ramp amplitude v ramp 1.25 vpp ` error amplifier input resistance r in v out =v fb 20 k ? ` current sense current set i set v cs = v cc ?0.4v 40 45 a v trip reference to v cc 40 45 a current sense delayed t csd 1.1 sec ` output drivers drive rise time, fall time t rf c l =3000pf 50 ns drive high v dh i source =20ma, v c1 =12v 10 11 v drive low v dl i sink =20ma, v c1 =12v 0.1 0.2 v ` uvlo and soft-start (ss) v cc5 start-up threshold v st v c1 > 4.0v 4.0 4.25 4.5 v hysteresis 0.10 v ss resistor r ss 20 k ? ss output enable v en 0.25 0.3 0.35 v hiccup duty cycle dc hic c ss = 0.1f, f req =100hz 10 % ` supply current v cc12 dynamic supply current i cd out freq = 200khz, c l =3000pf, synch., v ss > 0.3v 24 28 ma static supply current 12cv i vc1 v ss < 0.3v 5 7 ma 5v i vcc v ss > 0.3v 10 12 ma e e l l e e c c t t r r i i c c a a l l s s
microsemi linfinity microelectronics division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 4 copyright ? 2000 rev. 1.1,2003-09-04 www. microsemi . com lx1681/1682 v olta g e-mode pwm controllers p roduc tion a microsemi company l infinity theory of operation general description the lx1681/82 are voltage-mode pulse-width modulation controller integrated circuits. the internal oscillator and ramp generator frequency is fixed at 2 00khz. the devices have internal compensation, so that no exte rnal compensation is required. power up and initialization at power up, the lx1681/82 mon itors the supply voltage to both the +5v and the +12v pins (t here is no special requirement for the sequence of the two supplies). before both supplies reach their under-voltage lock-out (uvlo) thresholds, the soft-start (ss) pin is held low to prevent soft-s tart from beginning; the oscillator control is disabled and the top mosfet is kept off. soft-start once the supplies are above the uvl o threshold, the soft-start capacitor begins to be charged up by the reference through a 20k internal resistor. the capacitor voltage at the ss pin rises as a simple rc circuit. the ss pin is connected to the amplifier's non- inverting input that controls th e output voltage. the output voltage will follow the ss pin voltage if sufficient charging current is provided to the output capacitor. the simple rc soft-start allows the output to rise faster at the beginning and slower at the end of the soft-start interval. thus, the required charging current into the output capacitor is less at the end of the soft-start interval so decreasing the possibility of an over-current. a comparator monitors the ss pin voltage and indicates the end of soft-start when ss pin voltage reaches 95% of v ref . over-current protection (ocp) and hiccup the lx1681/1682 family uses the r ds(on) of the upper mosfet, together with a resistor (r set ) to set the actual current limit point. the comparator senses the current 1s after the top mosfet is switched on. experi ments have shown that the mosfet drain voltage will ring for 200-500ns after the gate is turned on. in order to reduce inaccuracies due to ringing, a 1s delay after gate turn-on is built in to the current sense comparator. the comparator draws a current (i set ), whose magnitude is 45a. the set resistor is selected to set the current limit for the application. when the sensed voltage across the r ds(on) plus the set resistor exceeds the 400mv v trip threshold, the ocp comparator outputs a signal to reset the pwm latch and to start hiccup mode. the soft-start capacitor (c ss ) is discharged slowly (10 times slower than when being charged up by r ss ). when the voltage on the ss/enable pin reaches a 0.3v threshold, hiccup finishes and the circuit soft-start s again. during hiccup, the top mosfet is off and the bottom mosfet remains on. hiccup is disabled during the soft-start inte rval, allowing the circuit to start up with the maximum current. if the rise speed of the output voltage is too fast, the required charging current to the output capacitor may be higher than the lim it-current. in this case, the peak mosfet current is regulated to the limit-current by the current-sense comparator. if the mosfet current still reaches its limit after the soft-start finishes, the hiccup is triggered again. the hiccup ensures the average heat generation on both mosfet?s and the average current to be much less than that in normal operation, if the output has a short circuit. over-current protection can also be implemented using a sense resi stor, instead of using the r ds(on) of the upper mosfet, for greater set-point accuracy. see application information section. oscillator frequency an internal oscillator sets the switching frequency at 200 khz. d d e e s s c c r r i i p p t t i i o o n n
microsemi linfinity microelectronics division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 5 copyright ? 2000 rev. 1.1,2003-09-04 www. microsemi . com lx1681/1682 v olta g e-mode pwm controllers p roduc tion a microsemi company l infinity application information output inductor the output inductor should be sele cted to meet the requirements of the output voltage ripple in steady-state operation and the inductor current slew-rate during transient. the peak-to-peak output voltage ripple is: ripple ripple i esr v = where () in out out in ripple v v v - v i = l f sw i ripple is the inductor ripple current, l is the output inductor value and esr is the effective series resistance of the output capacitor. i ripple should typically be in the range of 20% to 40% of the maximum output current. higher inductance results in lower output voltage ripple, allowing s lightly higher esr to satisfy the transient specification. higher i nductance also slows the inductor current slew rate in response to the load-current step change, ? i, resulting in more output-capacitor voltage droop. the inductor- current rise and fall times are: () out in v v ? ? = i l t rise and out v i l t fall ? = when using electrolytic capacitor s, the capacitor voltage droop is usually negligible, due to the large capacitance. output capacitor the output capacitor is sized to meet ripple and transient performance specifications. effective series resistance (esr) is a critical parameter. when a step load current occurs, the output voltage will have a step that equa ls the product of the esr and the current step, ? i. in an advanced microprocessor power supply, the output capacitor is usuall y selected for esr instead of capacitance or rms current capability. a capacitor that satisfies the esr requirement usually has a larger capacitance and current capability than strictly needed. the allowed esr can be found by: () ex v i esr < ? + ripple i where i ripple is the inductor ripple current, ? i is the maximum load current step change, and v ex is the allowed output voltage excursion in the transient. output capacitor (continued) electrolytic capacitors can be used for the output capacitor, but are less stable with age than tantalum capacitors. as they age, their esr degrades, reducing the system performance and increasing the risk of failure. it is recommended that multiple parallel capacitors be used, so that, as esr increases with age, overall performance will still meet the processor?s requirements. there is frequently strong pressure to use the least expensive components possible, however, this could lead to degraded long- term reliability, especially in the case of filter capacitors. linfinity?s demonstration boa rds use sanyo mv-gx filter capacitors, which are aluminum electrolytic, and have demonstrated reliability. the os con series from sanyo generally provides the very best performan ce in terms of long term esr stability and general reliability, but at a substantial cost penalty. the mv-gx series provides excellent esr performance at a reasonable cost. beware of off-b rand, very low-cost filter capacitors, which have been shown to degrade in both esr and general electrolytic characteristics over time. input capacitor the input capacitor and the input inductor are to filter the pulsating current generated by the buck converter to reduce interference to other circuits connected to the same 5v rail. in addition, the input capacitor provides local de-coupling the buck converter. the capacitor should be rated to handle the rms current requirement. the rms current is: ) 1 ( d d i i l rms ? = where i l is the inductor current and the d is the duty cycle. the maximum value, when d = 50%, i rms = 0.5 i l . for 5v input and output in the range of 2 to 3v, the required rms current is very close to 0.5 i l . soft-start capacitor the value of the soft-start capacitor determines how fast the output voltage rises and how large the inductor current is required to charge the output capacitor. the output voltage will follow the voltage at ss pin if the required inductor current does not exceed the maximum current in the inductor. the ss pin voltage can be expressed as: ) 1 ( / ss ss c r t set ss e v v ? ? = where v set is the reference voltage. r ss and c ss are soft start resistor and capacitor. the required inductor current for the output capacitor to follow the ss-pin voltage equals the required capacitor current plus the load current. th e soft-start capacitor should be selected so that the overall inductor current does not exceed it maximum. a a p p p p l l i i c c a a t t i i o o n n
microsemi linfinity microelectronics division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 6 copyright ? 2000 rev. 1.1,2003-09-04 www. microsemi . com lx1681/1682 v olta g e-mode pwm controllers p roduc tion a microsemi company l infinity application information soft-start capacitor (continued) the capacitor current to follow the ss-pin voltage is: () ss ss out c r t ss out out c e c c dt dv c i / ? = = where c out is the output capacitance. the typical value of c ss should be in the range of 0.1 to 0.2f. during the soft-start interval the load current from a micro- processor is negligible; therefore, the capacitor current is approximately the required inductor current. over-current protection current limiting occurs at current level i cl , when the voltage detected by the current sense comparator is greater than the current sense comparator threshold, v trip (400mv). trip set set on ds cl v r i r i = + ) ( so, a r i mv r i r i v r on ds cl set set on ds cl trip set 45 400 ) ( ) ( ? = ? = example: for 10a current limit, using irl3303 mosfet (26m ? r ds(on) ): ? = ? = ? k r set 1 . 3 10 45 026 . 0 10 4 . 0 6 current sensing using sense resistor the method of current sensing using the r ds(on) of the upper mosfet is economical, but can have a large tolerance, since the r ds(on) can vary with temperature, etc. a more accurate alternative is to use an external sense resistor (r sense ). since one input to the current sense comparator is th e supply voltage to the ic (v cc - pin 8), the sense resistor could be a pcb trace (for construction details, see application note an-10 or lx1668 data sheet). the over- current trip point is calculated as in the equations above, replacing r ds(on) with r sense . example: for 10a current limit, using a 5 sense resistor: ? = ? = ? = ? k r i r i v r set set sense cl trip set 8 . 7 10 45 005 . 0 10 4 . 0 ) ( 6 output enable the lx1681/82 fet driver outputs are driven to ground by pulling the soft-start pin below 0.3v. programming the output voltage the output voltage is sensed by the feedback pin (v fb ) which has a 1.25v reference. the output vo ltage can be set to any voltage above 1.25v (and lower than the input voltage) by means of a resistor divider (see product highlight). ) 1 ( 2 1 r r v v ref out + = note: keep r 1 and r 2 close to 10 0 (order of magnitude ). fet selection to insure reliable operation, the operating junction temperature of the fet switches must be kept below certain limits. the intel specification states that 115c maximum junction temperature should be maintained with an ambi ent of 50c. this is achieved by properly derating the part, and by adequate heat sinking. one of the most critical parameters for fet selection is the r ds(on) resistance. this parameter directly contribute s to the power dissipation of the fet devices, and thus impacts heat sink design, mechanical layout, and reliability. in general, the larger the current handling capability of the fet, the lower the r ds(on) will be, since more die area is available. this table gives selection of suitable fets from international rectifier. device r ds(on) @10v(m ? ) i d @ t c =100oc max. break- down voltage irl3803 6 83 30 irl22203n 7 71 30 irl3103 14 40 30 irl3102 13 56 20 irl3303 26 24 30 irl2703 40 17 30 all devices in to-220 package. for surface mount devices (to-263 / d 2 -pak), add 's' to part number, e.g. irl3103s. table 1 - fet selection guide heat dissipated in upper mosfet the heat dissipated in the top mosfet will be: ) 5 . 0 ( ) ( ) ( 2 s sw in on ds d f t v i cycle duty r i p + = where t sw is switching transition line for body diode (~100ns) and f s is the switching frequency. for the irl3102 (13 r ds(on) ), converting 5v to 2.0v at 15a will result in typical heat dissipation of 1.92w. a a p p p p l l i i c c a a t t i i o o n n
microsemi linfinity microelectronics division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 7 copyright ? 2000 rev. 1.1,2003-09-04 www. microsemi . com lx1681/1682 v olta g e-mode pwm controllers p roduc tion a microsemi company l infinity application information fet selection (continued) synchronous rectification ? lower mosfet the lower pass element can be either a mosfet or a schottky diode. the use of a mosfet (synchronous rectification) will result in higher efficiency, but at higher cost than using a schottky diode (non-synchronous). power dissipated in the bottom mosfet will be: [] w cycle duty r i p on ds d 51 . 3 1 ) ( 2 = ? = [irl3303 or 1.76w for the irl3102] non-synchronous operation - schottky diode a typical schottky diode, with a forward drop of 0.6v will dissipate 0.6 * 15 * [1 ? 2/5] = 5. 4w (compared to the 1.8 to 3.5w dissipated by a mosfet under the sa me conditions). this power loss becomes much more significant at lower duty cycles. the use of a dual schottky diode in a single to-220 package (e.g. the mbr2535) helps improve thermal dissipation. operation from a single power supply the lx1681/1682 needs a secondary supply voltage (v c1 ) to provide sufficient drive to the upper mosfet. in many applications with a 5v (v cc ) and a 12v (v c1 ) supply are present. in situations where only 5v is present, v c1 can be generated using a bootstrap (charge pump) circuit, as shown in figure 4 (typical applications section). the capacitor (c 4 ) is alternatively charged up from v cc via the schottky diode (d 2 ), and then boosted up when the fet is turned on. this scheme provides a v c1 voltage equal to 2 * v cc - v ds (d 2 ), or approximately 9.5v with v cc = 5v. this voltage will provide sufficient gate drive to the external mosfet in order to get a low r ds(on) . note that using the bootstrap circuit in synchronous r ectification mode is likely to result in faster turn-on than in non-synchronous mode. layout guidelines - thermal design a great deal of time and effort were spent optimizing the thermal design of the demonstration boards. any user who intends to implement an embedded mother board would be well advised to carefully read and follow these guidelines. if the fet switches have been carefully selected, exte rnal heatsinking is generally not required. however, this means th at copper trace on the pc board must now be used. this is a poten tial trouble spot; as much copper area as possible must be dedicated to heatsinking the fet switches , and the diode as well if a non-synchronous solution is used. in our vrm module, heatsi nk area was taken from internal ground and v cc planes which were actually split and connected with vias to the power devi ce tabs. the to-220 and to-263 cases are well suited for this application, and are the preferred packages. remember to remove any conformal coating from all exposed pc traces which are involved in heatsinking. lx168x 5v input output gnd figure 2 ? enabling linear regulator general notes as always, be sure to provide local capacitive decoupling close to the chip. be sure use ground plane construction for all high- frequency work. use low esr capacitors where justified, but be alert for damping and ringing pr oblems. high-frequency designs demand careful routing and layout, and may require several iterations to achieve desired performance levels. power traces to reduce power losses due to ohmic resistance, careful consid- eration should be given to the layout of traces that carry high currents. the main paths to consider are: ? input power from 5v supply to drain of top mosfet. ? trace between top mosfet and lower mosfet or schottky diode. ? trace between lower mosfet or schottky diode and ground. ? trace between source of t op mosfet and inductor and load. all of these traces should be made as wide and thick as possible, in order to minimize resistance a nd hence power losses. it is also recommended that, whenever possible, the ground, input and output power signals should be on separate planes (pcb layers). see figure 2 ? bold traces are power traces. layout assistance please contact linfinity?s applications engineers for assistance with any layout or component sele ction issues. a gerber file with layout for the most popular devices is available upon request. evaluation boards are also avai lable upon request. please check linfinity's web site for further application notes. a a p p p p l l i i c c a a t t i i o o n n
microsemi linfinity microelectronics division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 8 copyright ? 2000 rev. 1.1,2003-09-04 www. microsemi . com lx1681/1682 v olta g e-mode pwm controllers p roduc tion a microsemi company l infinity typical application lx1681 0.1f q 1 d 2 l 1 c 2 r 1 r 2 r set r sense c 1 c 3 v fb c ss n.c. gnd tdrv v c1 cs v c2 v boost 12v v in 5v v out ss lx1682 0.1f q 1 d 2 l 1 c 2 r 1 r 2 r set c 1 v fb c ss bdrv gnd tdrv v c1 cs v c2 v in 5v v out ss q 2 c 4 a a p p p p l l i i c c a a t t i i o o n n
microsemi linfinity microelectronics division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 9 copyright ? 2000 rev. 1.1,2003-09-04 www. microsemi . com lx1681/1682 v olta g e-mode pwm controllers p roduc tion a microsemi company l infinity block diagram cs comp + - amplifier/ compensation + - error comp + - i reset r s q q 5 v trip v cc 7 320k 20k 1 r ss v ref hiccup ramp oscillator 4 3 uvlo 8 2 6 l r 1 esr c out r 2 c in +12v v in (5v) +5v v core r set c ss v cc gnd bdrv tdrv v c1 cs v fb ss/enable hiccup uvlo i set pwm set v reset i set physical dimensions dm 8-pin plastic soic a p g b k l f c d j m * lead coplanarity note: 1. dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm(.006?) on any side. lead dimension shall not include solder coverage. m illimeters i nches dim min max min max a 4.83 5.00 0.190 0.197 b 3.81 3.94 0.150 0.155 c 1.35 1.75 0.053 0.069 d 0.33 0.51 0.013 0.020 f - 0.77 - 0.030 g 1.27 bsc 0.050 bsc j 0.19 0.25 0.007 0.010 k 0.13 0.25 0.005 0.010 l 4.80 5.21 0.189 0.205 m - 8 - 8 p 5.79 6.20 0.228 0.244 *lc - 0.10 - 0.004 b b l l o o c c k k d d i i a a g g r r a a m m
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