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LTC3632 1 3632f typical application features applications description high ef ciency, high voltage 20ma synchronous step-down converter the ltc ? 3632 is a high ef? ciency step-down dc/dc converter with internal high side and synchronous power switches that draws only 12a typical dc supply current at no load while maintaining output voltage regulation. the LTC3632 can supply up to 20ma load current and features a programmable peak current limit that provides a simple method for optimizing ef? ciency in lower current applications. the LTC3632s combination of burst mode ? operation, integrated power switches, low quiescent cur- rent, and programmable peak current limit provides high ef? ciency over a broad range of load currents. with its wide 4.5v to 50v input range and internal overvoltage monitor capable of protecting the part through 60v surges, the LTC3632 is a robust converter suited for regulating a wide variety of power sources. additionally, the LTC3632 includes a precise run threshold and soft-start feature to guarantee that the power system start-up is well-controlled in any environment. the LTC3632 is available in the thermally enhanced 3mm 3mm dfn and ms8e packages. ef? ciency and power loss vs load current n wide input voltage range: operation from 4.5v to 50v n overvoltage lockout provides protection up to 60v n internal high side and low side power switches n no compensation required n 20ma output current n low dropout operation: 100% duty cycle n low quiescent current: 12a n wide output voltage range: 0.8v to v in n 0.8v 1% feedback voltage reference n adjustable peak current limit n internal and external soft-start n precise run pin threshold with adjustable hysteresis n few external components required n low pro? le (0.75mm) 3mm 3mm dfn and thermally-enhanced ms8e packages n 4ma to 20ma current loops n industrial control supplies n distributed power systems n portable instruments n battery-operated devices n automotive power systems l , lt, ltc, ltm and burst mode are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. 5v, 20ma step-down converter v in LTC3632 run hyst 3632 ta01a sw v in 5v to 50v 1f 10f 1.47m 280k v out 5v 20ma v fb ss i set gnd 1mh load current (ma) 0.1 70 efficiency (%) power loss (mw) 80 90 100 110 3632 ta01b 60 50 40 30 20 10 100 1000 1 efficiency power loss v in = 10v v in = 48v
LTC3632 2 3632f absolute maximum ratings v in supply voltage ..................................... C0.3v to 60v sw voltage (dc) ............................C0.3v to (v in + 0.3v) run voltage .............................................. C0.3v to 60v v fb , hyst, i set , ss voltages......................... C0.3v to 6v operating junction temperature range (note 2) .................................................. C40c to 125c (note 1) 1 2 3 4 sw v in i set ss 8 7 6 5 gnd hyst v fb run top view 9 ms8e package 8-lead plastic msop t jmax = 125c, ja = 40c/w, jc = 5-10c/w exposed pad (pin 9) is gnd, must be soldered to pcb top view 9 dd package 8-lead (3mm s 3mm) plastic dfn 5 6 7 8 4 3 2 1 sw v in i set ss gnd hyst v fb run t jmax = 125c, ja = 43c/w, jc = 3c/w exposed pad (pin 9) is gnd, must be soldered to pcb pin configuration order information storage temperature range ................... C65c to 150c lead temperature (soldering, 10 sec) ms8e ................................................................ 300c lead free finish tape and reel part marking* package description temperature range LTC3632ems8e#pbf LTC3632ems8e#trpbf ltffz 8-lead plastic msop C40c to 125c LTC3632ims8e#pbf LTC3632ims8e#trpbf ltffz 8-lead plastic msop C40c to 125c LTC3632edd#pbf LTC3632edd#trpbf lfgb 8-lead (3mm 3mm) plastic dfn C40c to 125c LTC3632idd#pbf LTC3632idd#trpbf lfgb 8-lead (3mm 3mm) plastic dfn C40c to 125c consult ltc marketing for parts speci? ed with wider operating temperature ranges. *the temperature grade is identi? ed by a label on the shipping container. consult ltc marketing for information on non-standard lead based ? nish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/
LTC3632 3 3632f electrical characteristics the l denotes the speci? cations which apply over the full operating junction temperature range, otherwise speci? cations are at t a = 25c. v in = 10v, unless otherwise noted. symbol parameter conditions min typ max units input supply (v in ) v in input voltage operating range 4.5 50 v uvlo v in undervoltage lockout v in rising v in falling hysteresis l l 3.80 3.75 4.15 4.00 150 4.50 4.35 v v mv ovlo v in overvoltage lockout v in rising v in falling hysteresis 54 52 56 54 2 59 57 v v v i q dc supply current (note 3) active mode sleep mode shutdown mode v run = 0v 125 12 3 220 22 6 a a a output supply (v fb ) v fb feedback comparator trip voltage v fb rising l 0.792 0.800 0.808 v v hyst feedback comparator hysteresis l 357 mv i fb feedback pin current v fb = 1v C10 0 10 na v linereg feedback voltage line regulation v in = 4.5v to 50v 0.001 %/v operation v run run pin threshold run rising run falling hysteresis 1.17 1.06 1.21 1.10 110 1.25 1.14 v v mv i run run pin leakage current run = 1.3v C10 0 10 na v hystl hysteresis pin voltage low run < 1v, i hyst = 1ma 0.07 0.1 v i hyst hysteresis pin leakage current v hyst = 1.3v C10 0 10 na i ss soft-start pin pull-up current v ss < 1.5v 4.5 5.5 6.5 a t intss internal soft-start time ss pin floating 0.75 ms i peak peak current trip threshold i set floating 500k resistor from i set to gnd i set shorted to gnd l 40 8 50 25 10 60 13 ma ma ma r on power switch on-resistance top switch bottom switch i sw = C10ma i sw = 10ma 5.0 2.5 note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: the LTC3632e is guaranteed to meet speci? cations from 0c to 85c with speci? cations over the C40c to 125c operating junction temperature range assured by design, characterization and correlation with statistical process controls. the LTC3632i is guaranteed to meet speci? cations over the full C40c to 125c operating junction temperature range. t j is calculated from the ambient temperature t a and power dissipation p d according to the following formula: t j = t a + (p d ? ja c/w) note 3: dynamic supply current is higher due to the gate charge being delivered at the switching frequency. see applications information.
LTC3632 4 3632f typical performance characteristics ef? ciency vs load current, v out = 5v ef? ciency vs input voltage line regulation load regulation feedback comparator trip voltage vs temperature feedback comparator hysteresis vs temperature peak current trip threshold vs temperature load current (ma) 0.1 70 efficiency (%) 75 80 85 90 110 3632 g01 65 60 55 50 95 100 v in = 12v v in = 24v v in = 48v v out = 5v figure 10 circuit v in = 36v ef? ciency vs load current, v out = 3.3v load current (ma) 0.1 70 efficiency (%) 75 80 85 90 110 3632 g02 65 60 55 50 95 100 v in = 12v v in = 24v v in = 48v v out = 3.3v figure 10 circuit v in = 36v ef? ciency vs load current, v out = 2.5v load current (ma) 0.1 70 efficiency (%) 75 80 85 90 110 3632 g03 65 60 55 50 95 100 v in = 12v v in = 24v v in = 48v v out = 2.5v figure 10 circuit v in = 36v input voltage (v) 10 70 efficiency (%) 75 80 85 90 20 30 40 50 3632 g04 95 100 15 25 35 45 v out = 5v figure 10 circuit i load = 20ma i load = 5ma i load = 1ma input voltage (v) 5 C0.50 $ v out /v out (%) C0.40 C0.20 C0.10 0 0.50 0.20 15 25 30 50 3632 g05 C0.30 0.30 0.40 0.10 10 20 35 40 45 i load = 20ma figure 10 circuit load current (ma) 0 4.95 output voltage (v) 4.96 4.98 4.99 5.00 5.05 5.02 5 10 3632 g06 4.97 5.03 5.04 5.01 15 20 v in = 10v v out = 5v figure 10 circuit temperature (c) C40 0.798 feedback comparator trip voltage (v) 0.799 0.800 0.801 C10 20 50 80 3632 g07 110 v in = 10v temperature (c) C40 4.4 feedback comparator hysteresis (mv) 4.6 4.8 5.0 5.2 5.6 C10 20 50 80 3632 g08 110 5.4 v in = 10v temperature (c) C40 0 peak current trip threshold (ma) 10 20 30 40 60 C10 20 50 80 3632 g09 110 50 i set open i set = gnd r iset = 500k v in = 10v
LTC3632 5 3632f typical performance characteristics peak current trip threshold vs r iset quiescent supply current vs input voltage quiescent supply current vs temperature switch on-resistance vs input voltage switch on-resistance vs temperature switch leakage current vs temperature operating waveforms peak current trip threshold vs input voltage r iset (k) 0 0 peak current trip threshold (ma) 10 20 30 40 60 200 400 600 800 3632 g10 1000 1200 50 v in = 10v input voltage (v) 0 peak current trip threshold (ma) 20 40 60 10 30 50 10 20 30 40 3632 g11 50 5 015253545 i set open i set = gnd r set = 500k v in = 10v input voltage (v) 5 10 12 14 sleep 45 3632 g12 8 6 15 25 35 4 2 0 v in supply current (a) shutdown temperature (c) C40 10 12 14 sleep 110 3632 g13 8 6 C10 20 50 80 4 2 0 v in supply current (a) shutdown v in = 10v input voltage (v) 0 6 7 8 40 3632 g14 5 4 10 20 top bottom 30 50 3 2 1 switch on-resistance () temperature (c) switch on-resistance () 5 6 7 3632 g15 3 0 C40 C10 20 50 80 110 8 4 2 1 v in = 10v top bottom run comparator thresholds vs temperature temperature (c) 0 switch leakage current (a) 0.10 0.20 0.30 0.05 0.15 0.25 C10 20 50 3632 g16 C40 80 110 sw = 0v sw = 50v v in = 50v temperature (c) C40 1.00 run comparator threshold (v) 1.05 1.10 1.15 1.20 1.30 C10 20 50 80 3632 g17 110 1.25 rising falling switch voltage 20v/div output voltage 100mv/div inductor current 50ma/div 20s/div v in = 48v v out = 5v 3632 g18
LTC3632 6 3632f pin functions sw (pin 1): switch node connection to inductor. this pin connects to the drains of the internal power mosfet switches. v in (pin 2): main supply pin. a ceramic bypass capacitor should be tied between this pin and gnd (pin 8). i set (pin 3): peak current set input. a resistor from this pin to ground sets the peak current trip threshold. leave ? oating for the maximum peak current (50ma). short this pin to ground for the minimum peak current (10ma). a 1a current is sourced out of this pin. ss (pin 4) : soft-start control input. a capacitor to ground at this pin sets the ramp time to full current output dur- ing start-up. a 5a current is sourced out of this pin. if left ? oating, the ramp time defaults to an internal 0.75ms soft-start. run (pin 5): run control input. a voltage on this pin above 1.2v enables normal operation. forcing this pin below 0.7v shuts down the LTC3632, reducing quiescent current to approximately 3a. v fb (pin 6): output voltage feedback. connect to an external resistive divider to divide the output voltage down for comparison to the 0.8v reference. hyst (pin 7): run hysteresis open-drain logic output. this pin is pulled to ground when run (pin 5) is below 1.2v. this pin can be used to adjust the run pin hysteresis. see applications information. gnd (pin 8): chip ground. exposed pad (pin 9): ground. must be soldered to pcb ground for optimal electrical and thermal performance. soft-start waveforms typical performance characteristics load step transient response short-circuit response output voltage 2v/div inductor current 20ma/div 1ms/div c ss = 0.047f v in = 10v v out = 5v 3632 g19 output voltage 100mv/div load current 20ma/div 1ms/div v in = 10v v out = 5v 3632 g20 output voltage 2v/div inductor current 20ma/div 200s/div v in = 10v v out = 5v 3632 g21
LTC3632 7 3632f block diagram C + 1 logic and shoot- through prevention peak current comparator sw v in ss voltage reference feedback comparator 5a 3632 bd r2 r1 c1 v out l1 reverse current comparator C + C + C + + 0.800v 4 run 1.2v 5 i set 3 hyst 7 gnd 9 gnd 8 v fb 6 1a 2 c2
LTC3632 8 3632f operation the LTC3632 is a step-down dc/dc converter with internal power switches that uses burst mode control, combin- ing low quiescent current with high switching frequency, which results in high ef? ciency across a wide range of load currents. burst mode operation functions by using short burst cycles to ramp the inductor current through the internal power switches, followed by a sleep cycle where the power switches are off and the load current is supplied by the output capacitor. during the sleep cycle, the LTC3632 draws only 12a of supply current. at light loads, the burst cycles are a small percentage of the total cycle time which minimizes the average supply current, greatly improving ef? ciency. main control loop the feedback comparator monitors the voltage on the v fb pin and compares it to an internal 800mv reference. if this voltage is greater than the reference, the comparator activates a sleep mode in which the power switches and current comparators are disabled, reducing the v in pin supply current to only 12a. as the load current discharges the output capacitor, the voltage on the v fb pin decreases. when this voltage falls 5mv below the 800mv reference, the feedback comparator trips and enables burst cycles. at the beginning of the burst cycle, the internal high side power switch (p-channel mosfet) is turned on and the inductor current begins to ramp up. the inductor current increases until either the current exceeds the peak current comparator threshold or the voltage on the v fb pin exceeds 800mv, at which time the high side power switch is turned off. the low side power switch (n-channel mosfet) is then turned on. the inductor current ramps down until the reverse current comparator trips, signaling that the current is close to zero. if the voltage on the v fb pin is still less than the 800mv reference, the high side power switch is turned on again and another cycle commences. the average current during a burst cycle will normally be greater than the average load current. for this architecture, the maximum average output current is equal to half of the peak current. the hysteretic nature of this control architecture results in a switching frequency that is a function of the input voltage, output voltage and inductor value. this behavior provides inherent short-circuit protection. if the output is shorted to ground, the inductor current will decay very slowly during a single switching cycle. since the high side switch turns on only when the inductor current is near zero, the LTC3632 inherently switches at a lower frequency during start-up or short-circuit conditions. start-up and shutdown if the voltage on the run pin is less than 0.7v, the LTC3632 enters a shutdown mode in which all internal circuitry is disabled, reducing the dc supply current to 3a. when the voltage on the run pin exceeds 1.2v, normal operation of the main control loop is enabled. the run pin comparator has 110mv of internal hysteresis, and therefore must fall below 1.1v to disable the main control loop. the hyst pin provides an added degree of ? exibility for the run pin operation. this open-drain output is pulled to ground whenever the run comparator is not tripped, signaling that the LTC3632 is not in normal operation. in applications where the run pin is used to monitor the v in voltage through an external resistive divider, the hyst pin can be used to increase the effective run comparator hysteresis. an internal 1ms soft-start function limits the ramp rate of the output voltage on start-up to prevent excessive input supply droop. if a longer ramp time and consequently less supply droop is desired, a capacitor can be placed from the ss pin to ground. the 5a current that is sourced out of this pin will create a smooth voltage ramp on the capacitor. if this ramp rate is slower than the internal 1ms soft-start, then the output voltage will be limited by the ramp rate (refer to block diagram)
LTC3632 9 3632f operation (refer to block diagram) on the ss pin instead. the internal and external soft-start functions are reset on start-up and after undervoltage or overvoltage event on the input supply. in order to ensure a smooth start-up transition in any application, the internal soft-start also ramps the peak inductor current from 10ma during its 1ms ramp time to the set peak current threshold. the external ramp on the ss pin does not limit the peak inductor current during start-up; however, placing a capacitor from the i set pin to ground does provide this capability. peak inductor current programming the offset of the peak current comparator nominally pro- vides a peak inductor current of 50ma. this peak inductor current can be adjusted by placing a resistor from the i set pin to ground. the 1a current sourced out of this pin through the resistor generates a voltage that is translated into an offset in the peak current comparator, which limits the peak inductor current. undervoltage and overvoltage lockout the LTC3632 implements a protection feature which dis- ables switching when the input voltage is not within the 4.5v to 50v operating range. if v in falls below 4v typical (4.35v maximum), an undervoltage detector disables switching. similarly, if v in rises above 55v typical (53v minimum), an overvoltage detector disables switching. when switching is disabled, the LTC3632 can safely sustain input voltages up to the absolute maximum rating of 60v. switching is enabled when the input voltage returns to the 4.5v to 50v operating range.
LTC3632 10 3632f applications information the basic LTC3632 application circuit is shown on the front page of this data sheet. external component selection is determined by the maximum load current requirement and begins with the selection of the peak current programming resistor, r iset . the inductor value l can then be determined, followed by capacitors c in and c out . peak current resistor selection the peak current comparator has a maximum current limit of 50ma nominally, which results in a maximum aver- age current of 25ma. for applications that demand less current, the peak current threshold can be reduced to as little as 10ma. this lower peak current allows the use of lower value, smaller components (input capacitor, output capacitor and inductor), resulting in lower input supply ripple and a smaller overall dc/dc converter. the threshold can be easily programmed with an ap- propriately chosen resistor (r iset ) between the i set pin and ground. the value of resistor for a particular peak current can be computed by using figure 1 or the follow- ing equation: r iset = i peak ? 21 ? 10 6 where 10ma < i peak < 50ma. the peak current is internally limited to be within the range of 10ma to 50ma. shorting the i set pin to ground programs the current limit to 10ma, and leaving it ? oating sets the current limit to the maximum value of 50ma. when selecting this resistor value, be aware that the maximum figure 1. r iset selection average output current for this architecture is limited to half of the peak current. therefore, be sure to select a value that sets the peak current with enough margin to provide adequate load current under all foreseeable operating conditions. inductor selection the inductor, input voltage, output voltage and peak cur- rent determine the switching frequency of the LTC3632. for a given input voltage, output voltage and peak current, the inductor value sets the switching frequency when the output is in regulation. a good ? rst choice for the inductor value can be determined by the following equation: l v fi v v out peak out in = ? ? ? ? ? ? ? ? ? ? ? ? ? ?C 1 the variation in switching frequency with input voltage and inductance is shown in the following two ? gures for typical values of v out . for lower values of i peak , multiply the frequency in figure 2 and figure 3 by 50ma/i peak . an additional constraint on the inductor value is the LTC3632s 100ns minimum on-time of the high side switch. therefore, in order to keep the current in the inductor well controlled, the inductor value must be chosen so that it is larger than l min , which can be computed as follows: l vt i min in max on min peak max = () () () ? where v in(max) is the maximum input supply voltage for the application, t on(min) is 100ns, and i peak(max) is the maximum allowed peak inductor current. although the above equation provides the minimum inductor value, higher ef? ciency is generally achieved with a larger inductor value, which produces a lower switching frequency. for a given inductor type, however, as inductance is increased dc resistance (dcr) also increases. higher dcr trans- lates into higher copper losses and lower current rating, both of which place an upper limit on the inductance. the recommended range of inductor values for small surface mount inductors as a function of peak current is shown in figure 4. the values in this range are a good compromise between the tradeoffs discussed above. for applications maximum load current (ma) 4 r iset (k) 300 900 1000 1100 8 12 14 3632 f01 100 700 500 200 800 0 600 400 6 10 16 20 18
LTC3632 11 3632f applications information figure 3. switching frequency for v out = 3.3v figure 2. switching frequency for v out = 5v figure 4. recommended inductor values for maximum ef? ciency where board area is not a limiting factor, inductors with larger cores can be used, which extends the recommended range of figure 4 to larger values. inductor core selection once the value for l is known, the type of inductor must be selected. high ef? ciency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of the more expensive ferrite cores. actual core loss is independent of core size for a ? xed inductor value but is very dependent of the inductance selected. as the inductance increases, core losses decrease. un- fortunately, increased inductance requires more turns of wire and therefore copper losses will increase. ferrite designs have very low core losses and are pre- ferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. ferrite core material saturates hard, which means that inductance collapses abruptly when the peak design current is exceeded. this results in an abrupt increase in inductor ripple current and consequently output voltage ripple. do not allow the core to saturate! different core materials and shapes will change the size/current and price/current relationship of an inductor. toroid or shielded pot cores in ferrite or permalloy ma- terials are small and do not radiate energy but generally cost more than powdered iron core inductors with similar characteristics. the choice of which style inductor to use mainly depends on the price vs size requirements and any radiated ? eld/emi requirements. new designs for surface mount inductors are available from coiltronics, coilcraft, tdk, toko, sumida and vishay. c in and c out selection the input capacitor, c in , is needed to ? lter the trapezoidal current at the source of the top high side mosfet. to prevent large ripple voltage, a low esr input capacitor sized for the maximum rms current should be used. approximate rms current is given by: ii v v v v rms out max out in in out =? () ?? 1 v in input voltage (v) 5 switching frequency (khz) 200 250 300 35 3632 f02 150 100 15 25 50 45 30 10 20 40 50 0 350 l = 220h l = 470h l = 1000h l = 2200h v out = 5v i set open peak inductor current (ma) 10 100 inductor value (h) 1000 10000 50 3632 f04 v in input voltage (v) 5 0 switching frequency (khz) 50 150 200 250 15 25 30 50 3632 f03 100 10 20 35 40 45 l = 220h l = 470h l = 1000h l = 2200h v out = 3.3v i set open
LTC3632 12 3632f applications information this formula has a maximum at v in = 2v out , where i rms = i out /2. this simple worst-case condition is com- monly used for design because even signi? cant deviations do not offer much relief. note that ripple current ratings from capacitor manufacturers are often based only on 2000 hours of life which makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. several capacitors may also be paralleled to meet size or height requirements in the design. the output capacitor, c out , ? lters the inductors ripple current and stores energy to satisfy the load current when the LTC3632 is in sleep. the output voltage ripple during a burst cycle is dominated by the output capacitor equivalent series resistance (esr) and can be estimated by the following equation: v vi esr out out peak 160 < ? where the lower limit of v out /160 is due to the 5mv feedback comparator hysteresis. the value of the output capacitor must be large enough to accept the energy stored in the inductor without a large change in output voltage. setting this voltage step equal to 1% of the output voltage, the output capacitor must be: cl i v out peak out > ? ? ? ? ? ? 50 2 ?? typically, a capacitor that satis? es the esr requirement is adequate to ? lter the inductor ripple. to avoid overheating, the output capacitor must also be sized to handle the ripple current generated by the inductor. the worst-case ripple current in the output capacitor is given by i rms = i peak /2. multiple capacitors placed in parallel may be needed to meet the esr and rms current handling requirements. dry tantalum, special polymer, aluminum electrolytic, and ceramic capacitors are all available in surface mount packages. special polymer capacitors offer very low esr but have lower capacitance density than other types. tantalum capacitors have the highest capacitance density but it is important only to use types that have been surge tested for use in switching power supplies. aluminum electrolytic capacitors have signi? cantly higher esr but can be used in cost-sensitive applications provided that consideration is given to ripple current ratings and long- term reliability. ceramic capacitors have excellent low esr characteristics but can have high voltage coef? cient and audible piezoelectric effects. the high quality factor (q) of ceramic capacitors in series with trace inductance can also lead to signi? cant ringing. using ceramic input and output capacitors higher value, lower cost ceramic capacitors are now be- coming available in smaller case sizes. their high ripple current, high voltage rating and low esr make them ideal for switching regulator applications. however, care must be taken when these capacitors are used at the input and output. when a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, v in . at best, this ringing can couple to the output and be mistaken as loop instability. at worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at v in large enough to damage the part. for applications with inductive source impedance, such as a long wire, a series rc network may be required in parallel with c in to dampen the ringing of the input supply. figure 5 shows this circuit and the typical values required to dampen the ringing. LTC3632 v in c in l in 3632 f05 4 ? c in r = l in c in figure 5. series rc to reduce v in ringing output voltage programming the output voltage is set by an external resistive divider according to the following equation: vv r r out =+ ? ? ? ? ? ? 08 1 1 2 .?
LTC3632 13 3632f applications information the resistive divider allows the v fb pin to sense a fraction of the output voltage as shown in figure 6. v fb LTC3632 gnd v out r2 3632 f06 r1 figure 6. setting the output voltage to minimize the no-load supply current, resistor values in the megohm range should be used. the increase in supply current due to the feedback resistors can be calculated from: i v rr v v vin out out in = + ? ? ? ? ? ? ? ? ? ? ? ? 12 ? run pin with programmable hysteresis the LTC3632 has a low power shutdown mode controlled by the run pin. pulling the run pin below 0.7v puts the LTC3632 into a low quiescent current shutdown mode (i q ~ 3a). when the run pin is greater than 1.2v, the controller is enabled. figure 7 shows examples of con- ? gurations for driving the run pin from logic. LTC3632 run 4.7m v in 3632 f07 LTC3632 run v supply figure 7. run pin interface to logic the run pin can alternatively be con? gured as a precise undervoltage lockout (uvlo) on the v in supply with a resistive divider from v in to ground. the run pin com- parator nominally provides 10% hysteresis when used in this method; however, additional hysteresis may be added with the use of the hyst pin. the hyst pin is an open- drain output that is pulled to ground whenever the run comparator is not tripped. a simple resistive divider can be used as shown in figure 8 to meet speci? c v in voltage requirements. speci? c values for these uvlo thresholds can be computed from the following equations: ri g v uvlo threshold v r r in sin ? ? ? . ? =+ ? ? ? ? ? 121 1 1 2 ? ? =+ + fall g v uvlo threshold v r rr in in ? ? ? . ? 110 1 1 2 3 3 ? ? ? ? ? ? the minimum value of these thresholds is limited to the internal v in uvlo thresholds that are shown in the electri- cal characteristics table. the current that ? ows through this divider will directly add to the shutdown, sleep and active current of the LTC3632, and care should be taken to minimize the impact of this current on the overall ef? ciency of the application circuit. resistor values in the megohm range may be required to keep the impact on quiescent shutdown and sleep currents low. be aware that the hyst pin cannot be allowed to exceed its absolute maximum rating of 6v. to keep the voltage on the hyst pin from exceeding 6v, the following relation should be satis? ed: v r rr r v in max () ? 3 12 3 6 ++ ? ? ? ? ? ? < the run pin may also be directly tied to the v in supply for applications that do not require the programmable undervoltage lockout feature. in this con? guration, switch- ing is enabled when v in surpasses the internal undervoltage lockout threshold. soft-start the internal 0.75ms soft-start is implemented by ramping both the effective reference voltage from 0v to 0.8v and the peak current limit set by the i set pin (10ma to 50ma). run LTC3632 hyst v in r2 r1 r3 3632 f08 figure 8. adjustable undervoltage lockout
LTC3632 14 3632f applications information to increase the duration of the reference voltage soft-start, place a capacitor from the ss pin to ground. an internal 5a pull-up current will charge this capacitor, resulting in a soft-start ramp time given by: tc v a ss ss = ? . 08 5 when the LTC3632 detects a fault condition (input supply undervoltage or overvoltage) or when the run pin falls below 1.1v, the ss pin is quickly pulled to ground and the internal soft-start timer is reset. this ensures an orderly restart when using an external soft-start capacitor. the duration of the 0.75ms internal peak current soft- start may be increased by placing a capacitor from the i set pin to ground. the peak current soft-start will ramp from 10ma to the ? nal peak current value determined by a resistor from i set to ground. a 1a current is sourced out of the i set pin. with only a capacitor connected between i set and ground, the peak current ramps linearly from 10ma to 50ma, and the peak current soft-start time can be expressed as: tc v a ss iset iset () ? . = 08 1 a linear ramp of peak current appears as a quadratic waveform on the output voltage. for the case where the peak current is reduced by placing a resistor from i set to ground, the peak current offset ramps as a decaying exponential with a time constant of r iset ? c iset . for this case, the peak current soft-start time is approximately 3 ? r iset ? c iset . unlike the ss pin, the i set pin does not get pulled to ground during an abnormal event; however, if the i set pin is ? oat- ing (programmed to 50ma peak current), the ss and i set pins may be tied together and connected to a capacitor to ground. for this special case, both the peak current and the reference voltage will soft-start on power-up and after fault conditions. the ramp time for this combination is c ss(iset) ? (0.8v/6a). ef? ciency considerations the ef? ciency of a switching regulator is equal to the output power divided by the input power times 100%. it is often useful to analyze individual losses to determine what is limiting the ef? ciency and which change would produce the most improvement. ef? ciency can be expressed as: ef? ciency = 100% C (l1 + l2 + l3 + ...) where l1, l2, etc. are the individual losses as a percent- age of input power. although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses: v in operating current and i 2 r losses. the v in operating current dominates the ef? ciency loss at very low load currents whereas the i 2 r loss dominates the ef? ciency loss at medium to high load currents. 1. the v in operating current comprises two components: the dc supply current as given in the electrical charac- teristics and the internal mosfet gate charge currents. the gate charge current results from switching the gate capacitance of the internal power mosfet switches. each time the gate is switched from high to low to high again, a packet of charge, dq, moves from v in to ground. the resulting dq/dt is the current out of v in that is typically larger than the dc bias current. 2. i 2 r losses are calculated from the resistances of the internal switches, r sw , and external inductor r l . when switching, the average output current ? owing through the inductor is chopped between the high side pmos switch and the low side nmos switch. thus, the series resistance looking back into the switch pin is a function of the top and bottom switch r ds(on) values and the duty cycle (dc = v out /v in ) as follows: r sw = (r ds(on)top )dc + (r ds(on)bot )(1 C dc) the r ds(on) for both the top and bottom mosfets can be obtained from the typical performance characteris- tics curves. thus, to obtain the i 2 r losses, simply add r sw to r l and multiply the result by the square of the average output current: i 2 r loss = i o 2 (r sw + r l )
LTC3632 15 3632f applications information other losses, including c in and c out esr dissipative losses and inductor core losses, generally account for less than 2% of the total power loss. thermal considerations the LTC3632 does not dissipate much heat due to its high ef? ciency and low peak current level. even in worst-case conditions (high ambient temperature, maximum peak current and high duty cycle), the junction temperature will exceed ambient temperature by only a few degrees. design example as a design example, consider using the LTC3632 in an application with the following speci? cations: v in = 24v, v out = 3.3v, i out = 20ma, f = 250khz. furthermore, as- sume for this example that switching should start when v in is greater than 12v and should stop when v in is less than 8v. first, calculate the inductor value that gives the required switching frequency: l v khz ma v v = ? ? ? ? ? ? ? ? ? ? ? ? ? 33 250 50 1 33 24 220 . ? ?C . h next, verify that this value meets the l min requirement. for this input voltage and peak current, the minimum inductor value is: l vns ma h min =? 24 100 50 48 ? therefore, the minimum inductor requirement is satis? ed, and the 220h inductor value may be used. next, c in and c out are selected. for this design, c in should be size for a current rating of at least: ima v v v v ma rms rms =? 20 33 24 24 33 17 ? . ? . C due to the low peak current of the LTC3632, decoupling the v in supply with a 1f capacitor is adequate for most applications. c out will be selected based on the esr that is required to satisfy the output voltage ripple requirement. for a 50mv output ripple, the value of the output capacitor esr can be calculated from: v out = 50mv 50ma ? esr a capacitor with a 1 esr satis? es this requirement. a 10f ceramic capacitor has signi? cantly less esr than 1. the output voltage can now be programmed by choosing the values of r1 and r2. choose r2 = 240k and calculate r1 as: r v v rk out 1 08 1 2 750 = ? ? ? ? ? ? = . C? the undervoltage lockout requirement on v in can be satis? ed with a resistive divider from v in to the run and hyst pins. choose r1 = 2m and calculate r2 and r3 as follows: r v vv rk r in rising 2 121 121 1 224 3 1 = ? ? ? ? ? ? = = . C. ? () .. C. ?C . () 1 11 12908 v vv rr k in falling ? ? ? ? ? ? = choose standard values for r2 = 226k and r3 = 91k. the i set pin should be left open in this example to select maximum peak current (50ma). figure 9 shows a complete schematic for this design example. v in LTC3632 run 2m 1f 226k 91k hyst 3632 f09 sw v in 24v v out 3.3v 20ma i set ss v fb gnd 750k 10f 220h 240k figure 9. 24v to 3.3v, 20ma regulator at 250khz
LTC3632 16 3632f applications information pc board layout checklist when laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3632. check the following in your layout: 1. large switched currents ? ow in the power switches and input capacitor. the loop formed by these compo- nents should be as small as possible. a ground plane is recommended to minimize ground impedance. 2. connect the (+) terminal of the input capacitor, c in , as close as possible to the v in pin. this capacitor provides the ac current into the internal power mosfets. 3. keep the switching node, sw, away from all sensitive small signal nodes. the rapid transitions on the switching node can couple to high impedance nodes, in particular v fb , and create increased output ripple. 4. flood all unused area on all layers with copper. flooding with copper will reduce the temperature rise of power components. you can connect the copper areas to any dc net (v in , v out , gnd or any other dc rail in your system). typical applications figure 10. high ef? ciency 5v regulator 3.3v, 20ma regulator with peak current soft-start, small size soft-start waveforms v in LTC3632 run c in 4.7f hyst 3632 f10a sw v in 5v to 50v v out 5v 20ma v fb ss i set gnd l1 1mh r1 4.2m r2 800k c in : tdk c5750x7r2a475mt c out : avx 1812d107mat l1: coilcraft lps6235-105ml c out 100f c ss 470nf output voltage 1v/div inductor current 20ma/div 2ms/div 3632 ta03b v in LTC3632 run c in 1f ss 3632 ta03a sw v in 4.5v to 24v v out 3.3v 20ma v fb hyst i set gnd l1 470h r1 294k r2 93.1k c in : tdk c3216x7r1e105kt c out : avx 08056d106kat2a l1: murata lqh43cn471k03 c out 10f c ss 0.1f
LTC3632 17 3632f typical applications positive-to-negative converter maximum load current vs input voltage v in LTC3632 run c in 1f hyst 3632 ta04a sw v in 4.5v to 38v v out C12v v fb ss i set gnd l1 1mh r1 1m r2 71.5k c in : tdk c3225x7r1h105kt c out : murata grm32dr71c106ka01 l1: tyco/coev dq6545-102m c out 10f v in input voltage (v) 5 5 maximum load current (ma) 15 10 20 15 25 30 50 3632 ta04b 10 20 35 40 45 i set open v out = C3v v out = C5v v out = C12v small size, limited peak current, 4ma regulator v in LTC3632 run c in 1f r3 470k r4 100k r5 33k i set 3632 ta05a sw v in 7v to 50v v out 5v 4ma v fb ss hyst gnd l1 2.2mh r1 470k r2 88.7k c in : tdk c3225x7r1h105kt c out : avx 08056d106kat2a l1: murata lqh43nn222k03 c out 10f v in LTC3632 run c in 1f ss 3642 ta07a sw v in 15v to 50v v out 15v 4ma v fb hyst i set gnd l1 10mh r1 3m r2 169k c in : avx 18125c105kat2a c out : tdk c3216x7r1e475kt l1: coilcraft lps6235-106ml c out 4.7f load current (ma) 0.1 50 efficiency (%) 55 65 75 85 14 3632 ta07b 95 60 70 80 90 v in = 24v v in = 36v v in = 48v high ef? ciency 15v, 4ma regulator ef? ciency vs load current
LTC3632 18 3632f package description 3.00 p 0.10 (4 sides) note: 1. drawing to be made a jedec package outline m0-229 variation of (weed-1) 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on top and bottom of package 0.38 p 0.10 bottom viewexposed pad 1.65 p 0.10 (2 sides) 0.75 p 0.05 r = 0.115 typ 2.38 p 0.10 (2 sides) 1 4 8 5 pin 1 top mark (note 6) 0.200 ref 0.00 C 0.05 (dd) dfn 1203 0.25 p 0.05 2.38 p 0.05 (2 sides) recommended solder pad pitch and dimensions 1.65 p 0.05 (2 sides) 2.15 p 0.05 0.50 bsc 0.675 p 0.05 3.5 p 0.05 package outline 0.25 p 0.05 0.50 bsc dd package 8-lead plastic dfn (3mm 3mm) (reference ltc dwg # 05-08-1698)
LTC3632 19 3632f information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. package description ms8e package 8-lead plastic msop, exposed die pad (reference ltc dwg # 05-08-1662 rev e) msop (ms8e) 0908 rev e 0.53 p 0.152 (.021 p .006) seating plane note: 1. dimensions in millimeter/(inch) 2. drawing not to scale 3. dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.152mm (.006") per side 4. dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.152mm (.006") per side 5. lead coplanarity (bottom of leads after forming) shall be 0.102mm (.004") max 0.18 (.007) 0.254 (.010) 1.10 (.043) max 0.22 C 0.38 (.009 C .015) typ 0.86 (.034) ref 0.65 (.0256) bsc 0 o C 6 o typ detail a detail a gauge plane 12 3 4 4.90 p 0.152 (.193 p .006) 8 8 1 bottom view of exposed pad option 7 6 5 3.00 p 0.102 (.118 p .004) (note 3) 3.00 p 0.102 (.118 p .004) (note 4) 0.52 (.0205) ref 1.83 p 0.102 (.072 p .004) 2.06 p 0.102 (.081 p .004) 5.23 (.206) min 3.20 C 3.45 (.126 C .136) 2.083 p 0.102 (.082 p .004) 2.794 p 0.102 (.110 p .004) 0.889 p 0.127 (.035 p .005) recommended solder pad layout 0.42 p 0.038 (.0165 p .0015) typ 0.65 (.0256) bsc 0.1016 p 0.0508 (.004 p .002) detail b detail b corner tail is part of the leadframe feature. for reference only no measurement purpose 0.05 ref 0.29 ref
LTC3632 20 3632f linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2009 lt 0509 ? printed in usa related parts typical application 5v, 20ma regulator for automotive applications v in LTC3632 run c in 1f ss 3632 ta06a sw v batt 4.5v to 50v transients up to 60v v out 5v 20ma v fb hyst i set gnd l1 1mh r1 470k r2 88.7k c in : tdk c3225x7r2a105m c out : kemet c1210c106k4rac l1: coiltronics dra73-102-r c out 10f part number description comments ltc1474 18v, 250ma (i out ), high ef? ciency step-down dc/dc converter v in : 3v to 18v, v out(min) = 1.2v, i q = 10a, i sd = 6a, msop8 lt ? 1766 60v, 1.2a (i out ), 200khz, high ef? ciency step-down dc/dc converter v in : 5.5v to 60v, v out(min) = 1.2v, i q = 2.5ma, i sd = 25a, tssop16/e lt1934/lt1934-1 36v, 250ma (i out ), micropower step-down dc/dc converter with burst mode operation v in : 3.2v to 34v, v out(min) = 1.25v, i q = 12a, i sd < 1a, thinsot ? package lt1936 36v, 1.4a (i out ), 500khz high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(min) = 1.2v, i q = 1.9ma, i sd < 1a, ms8e lt1939 25v, 2a, 2.5mhz high ef? ciency dc/dc converter and ldo controller v in : 3.6v to 25v, v out(min) = 0.8v, i q = 2.5ma, i sd < 10a, 3mm 3mm dfn10 lt3437 60v, 400ma (i out ), micropower step-down dc/dc converter with burst mode operation v in : 3.3v to 60v, v out(min) = 1.25v, i q = 100a, i sd < 1a, 3mm 3mm dfn10, tssop16e lt3470 40v, 250ma (i out ), high ef? ciency step-down dc/dc converter with burst mode operation v in : 4v to 40v, v out(min) = 1.2v, i q = 26a, i sd < 1a, 2mm 3mm dfn8, thinsot lt3480 36v with transient protection to 60v, 2a (i out ), 2.4mhz, high ef? ciency step-down dc/dc converter with burst mode operation v in : 3.6v to 38v, v out(min) = 0.78v, i q = 70a, i sd < 1a, 3mm 3mm dfn10, msop10e lt3493 36v, 1.4a (i out ), 750khz high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(min) = 0.8v, i q = 1.9ma, i sd < 1a, 2mm 3mm dfn6 lt3500 36v, 40v max , 2a, 2.5mhz high ef? ciency dc/dc converter and ldo controller v in : 3.6v to 36v, v out(min) = 0.8v, i q = 2.5ma, i sd < 10a, 3mm 3mm dfn10 lt3505 36v with transient protection to 40v, 1.4a (i out ), 3mhz, high ef? ciency step-down dc/dc converter v in : 3.6v to 34v, v out(min) = 0.78v, i q = 2ma, i sd = 2a, 3mm 3mm dfn8, msop8e lt3506/lt3506a 25v, dual 1.6a (i out ), 575khz/1.1mhz high ef? ciency step-down dc/dc converter v in : 3.6v to 25v, v out(min) = 0.8v, i q = 3.8ma, i sd = 30a, 5mm 4mm dfn16, tssop16e lt3508 36v with transient protection to 40v, dual 1.4a (i out ), 3mhz, high ef? ciency step-down dc/dc converter v in : 3.7v to 37v, v out(min) = 0.8v, i q = 4.6ma, i sd = 1a, 4mm 4mm qfn24, tssop16e ltc3631/ltc3631-3.3/ ltc3631-5 45v, 100ma synchronous micropower step-down dc/dc converter v in : 4.5v to 45v (60v max ), v out(min) = 0.8v, i q = 12a, i sd = 3a, 3mm 3mm dfn8, msop8e ltc3642/ltc3642-3.3/ ltc3642-5 45v, 50ma synchronous micropower step-down dc/dc converter v in : 4.5v to 45v (60v max ), v out(min) = 0.8v, i q = 12a, i sd = 3a, 3mm 3mm dfn8, msop8e lt3685 36v with transient protection to 60v, 2a (i out ), 2.4mhz, high ef? ciency step-down dc/dc converter v in : 3.6v to 38v, v out(min) = 0.78v, i q = 70a, i sd < 1a, 3mm 3mm dfn10, msop10e thinsot is a trademark of linear technology corporation.

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