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1 ltc1265/ltc1265-3.3/LTC1265-5 v in 5.4v to 12v l1* 33 m h r sense ** 0.1 w v out 5v 1a c out ?? 220 m f 10v 1000pf 130pf 3900pf 1k 0.1 m f c in ??? 68 m f 20v d1 ? pwr v in v in sw pgnd sense + sense sgnd i th c t ltc1265-fo1 shdn * ** ? ?? ??? coiltronics ctx33-4 irc lrc2010-01-r100-j mbrs130lt3 avx tpse227k010 avx tpse686k020 LTC1265-5 + + figure 1. high efficiency step-down converter 1.2a, high efficiency step-down dc/dc converter n high efficiency: up to 95% n current mode operation for excellent line and load transient response n internal 0.3 w power switch (v in = 10v) n short-circuit protection n low dropout operation: 100% duty cycle n low-battery detector n low 160 m a standby current at light loads n active-high micropower shutdown: i q < 15 m a n peak inductor current independent of inductor value n available in 14-pin so package the ltc ? 1265 is a monolithic step-down current mode dc/dc converter featuring burst mode tm operation at low output current. the ltc1265 incorporates a 0.3 w switch (v in =10v) allowing up to 1.2a of output current. under no load condition, the converter draws only 160 m a. in shutdown it typically draws a mere 5 m a making this converter ideal for current sensitive applications. in drop- out the internal p-channel mosfet switch is turned on continuously maximizing the life of the battery source. the ltc1265 incorporates automatic power saving burst mode operation to reduce gate charge losses when the load currents drop below the level required for continuous operation. the inductor current is user-programmable via an external current sense resistor. operation up to 700khz permits the use of small surface mount inductors and capacitors. n 5v to 3.3v conversion n distributed power systems n step-down converters n inverting converters n memory backup supply n portable instruments n battery-powered equipment n cellular telephones LTC1265-5 efficiency burst mode is a trademark of linear technology corporation. features descriptio u applicatio s u typical applicatio u load current (a) 0.01 70 efficiency (%) 75 80 85 90 100 0.10 1.00 ltc1265 ta01 95 v in = 6v v in = 9v v in = 12v l = 33 m h v out = 5v r sense = 0.1 w c t = 130pf , ltc and lt are registered trademarks of linear technology corporation.
2 ltc1265/ltc1265-3.3/LTC1265-5 the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v in = 10v, v shdn = 0v, unless otherwise specified. symbol parameter conditions min typ max units i fb feedback current into pin 9 ltc1265 0.2 1 m a v fb feedback voltage ltc1265c l 1.22 1.25 1.28 v v in = 9v, ltc1265i l 1.20 1.25 1.30 v v out regulator output voltage ltc1265-3.3: i load = 800ma l 3.22 3.3 3.40 v LTC1265-5: i load = 800ma l 4.9 5 5.2 v d v out output voltage line regulation v in = 6.5v to 10v, i load = 800ma C40 0 40 mv output voltage load regulation ltc1265-3.3: 10ma < i load < 800ma 40 65 mv LTC1265-5: 10ma < i load < 800ma 60 100 mv burst mode operation output ripple i load = 0ma 50 mv p-p i q input dc supply current (note 3) active mode: 3.5v < v in < 10v 1.8 2.4 ma sleep mode: 3.5v < v in < 10v 160 230 m a sleep mode: 5v < v in < 10v (LTC1265-5) 160 230 m a shutdown: v shdn = v in , 3.5v < v in < 10v 5 15 m a v lbtrip low-battery trip point 1.15 1.25 1.35 v i lbin current into pin 4 0.5 m a i lbout current sunk by pin 3 v lbout = 0.4v, v lbin = 0v 0.5 1.0 1.5 ma v lbout = 5v, v lbin = 10v 1.0 m a v 8 C v 7 current sense threshold voltage ltc1265: v sense C = 5v, v 9 = v out /4 + 25mv (forced) 25 mv v sense C = 5v, v 9 = v out /4 C 25mv (forced) 130 150 180 mv ltc1265-3.3: v sense C = v out + 100mv (forced) 25 mv v sense C = v out C 100mv (forced) 130 150 180 mv LTC1265-5: v sense C = v out + 100mv (forced) 25 mv v sense C = v out C 100mv (forced) 130 150 180 mv r on on resistance of switch ltc1265c l 0.3 0.60 w ltc1265i 0.3 0.70 w i 5 c t pin discharge current v out in regulation, v sense C = v out 40 60 100 m a v out = 0v 2 10 m a t off switch off time (note 4) c t = 390pf, i load = 800ma (ltc1265c) l 456 m s c t = 390pf, i load = 800ma (ltc1265i) l 3.5 5 7 m s input supply voltage (pins 1, 2, 13) ..........C 0.3v to 13v dc switch current (pin 14) .................................... 1.2a peak switch current (pin 14) ................................. 1.6a switch voltage (pin 14) .................................. v in C 13.0 operating temperature range ltc1265c ............................................... 0 to 70 c ltc1265i ........................................ C 40 c to 85 c junction temperature (note 2) ............................. 125 c storage temperature range ....................C 65 to 150 c lead temperature (soldering, 10 sec).................. 300 c (voltages refer to gnd pin) (note 1) order part number ltc1265cs ltc1265cs-5 ltc1265cs-3.3 ltc1265is consult factory for military grade parts. top view s package 14-lead plastic so *adjustable output version t jmax = 125 c, q ja = 110 c/w 1 2 3 4 5 6 7 14 13 12 11 10 9 8 pwr v in v in lb out lb in c t i th sense sw pwr v in pgnd sgnd shdn n/c (v fb *) sense + absolute axi u rati gs w ww u package/order i for atio uu w electrical characteristics
3 ltc1265/ltc1265-3.3/LTC1265-5 note 3: dynamic supply current is higher due to the gate charge being delivered at the switching frequency. note 4: in applications where r sense is placed at ground potential, the off time increases by approximately 40%. note 1: absolute maximum ratings are those values beyond which the life of the device may be impaired. note 2: t j is calculated from the ambient temperature t a and power dissipation p d according to the following formulas: ltc1265cs, ltc1265cs-3.3, ltc1265cs-5: t j = t a + (p d ? 110 c/w) load current (a) 0.01 70 efficiency (%) 75 80 85 90 100 0.10 1.00 1265 g01 95 v in = 5v v in = 9v v in = 12v ltc1265-3.3 v out = 3.3v r sense = 0.1 w c t = 130pf coil = ctx33-4 input voltage (v) 4 80 efficiency (%) 82 86 88 90 100 94 6 8 913 ltc1265 g03 84 96 98 92 57 10 11 12 i load = 250ma i load = 800ma ltc1265-3.3 r sense = 0.1 w c t = 130pf coil = ctx33-4 input voltage (v) 4 80 efficiency (%) 82 86 88 90 100 94 6 8 913 1265 g02 84 96 98 92 57 10 11 12 i load = 250ma i load = 800ma LTC1265-5 r sense = 0.1 w c t = 130pf coil = ctx33-4 switch resistance input voltage (v) 3 0 rds (on) ( ) 0.1 0.3 0.4 0.5 7 11 13 0.9 1265 g05 0.2 59 6 10 12 48 0.6 0.7 0.8 t j = 70 c t j = 125 c t j = 25 c t j = 0 c (v in v out ) voltage (v) 0 normalized frequency 0.4 0.8 1.2 1.0 0.6 0.2 2 1468 1265 g04 10 03579 0 c 25 c 70 c operating frequency vs (v in C v out ) efficiency vs input voltage (v out = 3.3v) efficiency vs input voltage (v out = 5v) efficiency vs load current switch leakage current temperature ( c) 0 leakage current (na) 180 240 300 270 210 150 90 30 80 1265 g06 120 60 0 20 40 60 100 v in = 12v symbol parameter conditions min typ max units v ih shutdown pin high min voltage at pin 10 for device to be in shutdown 1.2 v v il shutdown pin low max voltage at pin 10 for device to be active 0.6 v i 10 shutdown pin input current v shdn = 8v 0.5 m a the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v in = 10v, v shdn = 0v, unless otherwise specified. electrical characteristics typical perfor a ce characteristics uw
4 ltc1265/ltc1265-3.3/LTC1265-5 pwr v in (pins 1, 13): supply for the power mosfet and its driver. must decouple this pin properly to ground. must always tie pins 1 and 13 together. v in (pin 2): main supply for all the control circuitry in the ltc1265. lb out (pin 3): open-drain output of the low-battery comparator. this pin will sink current when pin 4 (lb in ) goes below 1.25v. during shutdown, this pin is high impedance. lb in (pin 4): the (C) input of the low-battery comparator. the (+) input is connected to a reference voltage of 1.25v. c t (pin 5): external capacitor c t from pin 5 to ground sets the switch off time. the operating frequency is dependent on the input voltage and c t . i th (pin 6): feedback amplifier decoupling point. the current comparator threshold is proportional to pin 6 voltage. sense C (pin 7): connect to the (C) input of the current comparator. for ltc1265-3.3 and LTC1265-5, it also connects to an internal resistive divider which sets the output voltage. sense + (pin 8): the (+) pin to the current comparator. a built-in offset between pins 7 and 8 in conjunction with r sense sets the current trip threshold. n/c,v fb (pin 9): for the ltc1265 adjustable version, this pin serves as the feedback pin from an external resistive divider used to set the output voltage. on the ltc1265-3.3 and LTC1265-5 versions, this pin is not used. shdn (pin 10): pulling this pin high keeps the internal switch off and puts the ltc1265 in micropower shut- down. do not float this pin. sgnd (pin 11): small-signal ground. must be routed separately from other grounds to the (C) terminal of c out . pgnd (pin 12): switch driver ground. connects to the (C) terminal of c in . anode of the schottky diode must be connected close to this pin. sw (pin 14): drain of the p-channel mosfet switch. cathode of the schottky diode must be connected close to this pin. dc supply current supply current in shutdown gate charge losses input voltage (v) 0 0 supply current (ma) 0.3 0.6 0.9 2.1 1.5 2 6 814 1.8 1.2 4 10 12 does not include gate charge active mode sleep mode 1265 g07 input voltage (v) 3 supply current ( a) 6 8 11 1265 g08 4 2 0 5 7 9 13 5 7 3 1 10 4 6 8 12 shutdown = 3v t a = 25c frequency (khz) 0 0 switching current (ma) 1.0 2.0 3.0 4.0 200 400 1265 g09 600 5.0 5.5 4.5 3.5 2.5 1.5 0.5 800 1000 v in = 12v v in = 9v v in = 6v typical perfor a ce characteristics uw uu u pi fu ctio s
5 ltc1265/ltc1265-3.3/LTC1265-5 (pin 9 connection shown for ltc1265-3.3 and LTC1265-5; change create ltc1265) the ltc1265 uses a constant off-time architecture to switch its internal p-channel power mosfet. the off time is set by an external timing capacitor at c t (pin 5). the operating frequency is then determined by the off time and the difference between v in and v out . the output voltage is set by an internal resistive divider (ltc1265-3.3 and LTC1265-5) connected to sense C (pin 7) or an external divider returned to v fb (pin 9 for ltc1265). a voltage comparator v, and a gain block g, compare the divided output voltage with a reference voltage of 1.25v. to optimize efficiency, the ltc1265 automatically switches between continuous and burst mode operation. the volt- age comparator is the primary control element when the device is in burst mode operation, while the gain block controls the output voltage in continuous mode. when the load is heavy, the ltc1265 is in continuous operation. during the switch on time, current comparator c monitors the voltage between pins 7 and 8 connected across an external shunt in series with the inductor. when the voltage across the shunt reaches the comparators threshold value, its output signal will change state, setting the flip flop and turning the internal p-channel mosfet off. the timing capacitor connected to pin 5 is now allowed to discharge at a rate determined by the off-time controller. when the voltage on the timing capacitor has discharged past v th1 , comparator t trips, sets the flip flop and causes the switch to turn on. also, the timing capacitor is re- charged. the inductor current will again ramp up until the current comparator c trips. the cycle then repeats. when the load current increases, the output voltage de- creases slightly. this causes the output of the gain stage (pin 6) to increase the current comparator threshold, thus tracking the load current. when the load is relatively light, the ltc1265 automati- cally goes into burst mode operation. the current loop is interrupted when the output voltage exceeds the desired regulated value. the hysteretic voltage comparator v trips when v out is above the desired output voltage, shutting off the switch and causing the capacitor to discharge. this (refer to functional diagram) pwr v in sense + sense 5pf 100k lb in sgnd i th v in sense v fb v th1 v th2 sleep pgnd sw 13k v os v fb adjustable version 25mv to 150mv off-time control shdn lb 0ut c t + v + c r s q + t + s + a3 reference + g 1265 fd 5 2 3 6 11 4 10 9 7 8 14 12 1, 13 fu ctio al diagra u u w operatio u
6 ltc1265/ltc1265-3.3/LTC1265-5 capacitor discharges past v th1 until its voltage drops below v th2 . comparator s then trips and a sleep signal is generated. the circuit now enters into sleep mode with the power mosfet turned off. in sleep mode, the ltc1265 is in standby and the load current is supplied by the output capacitor. all unused circuitry is shut off, reducing quies- cent current from 2ma to 160 m a. when the output capaci- tor discharges by the amount of the hysteresis of the comparator v, the p-channel switch turns on again and the process repeats itself. during burst mode operation the peak inductor current is set at 25mv/r sense . to avoid the operation of the current loop interfering with burst mode operation, a built-in offset v os is incorporated in the gain stage. this prevents the current from increas- ing until the output voltage has dropped below a minimum threshold. using constant off-time architecture, the operating fre- quency is a function of the voltage. to minimize the frequency variation as dropout is approached, the off-time controller increases the discharge current as v in drops below v out + 2v. in dropout the p-channel mosfet is turned on continuously (100% duty cycle) providing low dropout operation with v out @ v in . the basic ltc1265 application circuit is shown in figure 1. external component selection is driven by the load requirement, and begins with the selection of r sense . once r sense is known, c t and l can be chosen. next, the schottky diode d1 is selected followed by c in and c out . r sense selection for output current r sense is chosen based on the required output current. with the current comparator monitoring the voltage devel- oped across r sense , the threshold of the comparator determines the peak inductor current. depending on the load current condition, the threshold of the comparator lies between 25mv/r sense and 150mv/r sense . the maxi- mum output current of the ltc1265 is: i out(max) = 150mv r sense i ripple 2 (amps) where i ripple is the peak-to-peak inductor ripple current. at a relatively light load, the ltc1265 is in burst mode operation. in this mode the peak inductor current is set at 25mv/r sense . to fully benefit from burst mode operation, the inductor current should be continuous during burst periods. hence, the peak-to-peak inductor ripple current must not exceed 25mv/r sense . to account for light and heavy load conditions, the i out(max) is then given by: i out(max) = 150mv r sense 25mv 2 ?r sense 137.5mv r sense = (amps) (amps) solving for r sense and allowing a margin of variations in the ltc1265 and extended component values yields: r sense = 100mv i out(max) ( w ) the ltc1265 is rated with a capability to supply a maximum of 1.2a of output current. therefore, the minimum value of r sense that can be used is 0.083 w . a graph for selecting r sense versus maximum output is given in figure 2. figure 2. selecting r sense (refer to functional diagram) maximum output current (a) 0 r sense ( w ) 0.3 0.4 0.5 0.8 1265 g10 0.2 0.1 0 0.2 0.4 0.6 1 operatio u applicatio s i for atio wu uu
7 ltc1265/ltc1265-3.3/LTC1265-5 kool m m is a registered trademark of magnetics, inc. under short-circuit condition, the peak inductor current is determined by: i sc(pk) = 150mv r sense (amps) in this condition, the ltc1265 automatically extends the off time of the p-channel mosfet to allow the inductor current to decay far enough to prevent any current build- up. the resulting ripple current causes the average short- circuit current to be approximately i out(max) . c t and l selection for operating frequency the ltc1265 uses a constant off-time architecture with t off determined by an external capacitor c t . each time the p-channel mosfet turns on, the voltage on c t is reset to approximately 3.3v. during the off time, c t is discharged by a current that is proportional to v out . the voltage on c t is analogous to the current in inductor l, which likewise, decays at a rate proportional to v out . thus the inductor value must track the timing capacitor value. the value of c t is calculated from the desired continuous mode operating frequency: c t = 1 1.3(10 4 )f v in ?v out v in + v d (farads) ) ) where v d is the drop across the schottky diode. as the operating frequency is increased, the gate charge losses will reduce efficiency. the complete expression for operating frequency is given by: f ? 1 t off v in ?v out v in + v d (hz) ) ) where: t off = 1.3(10 4 )c t v reg v out (sec) ) ) v reg is the desired output voltage (i.e. 5v, 3.3v). v out is the measured output voltage. thus v reg /v out = 1 in regulation. note that as v in decreases, the frequency decreases. when the input-to-output voltage differential drops below 2v, the ltc1265 reduces t off by increasing the discharge current in c t . this prevents audible operation prior to dropout. (see shelving effect shown in the operating frequency curve under typical performance character- istics.) to maintain continuous inductor current at light load, the inductor must be chosen to provide no more than 25mv/ r sense of peak-to-peak ripple current. this results in the following expression for l: l 3 5.2(10 5 )r sense (c t )v reg using an inductance smaller than the above value will result in the inductor current being discontinuous. a consequence of this is that the ltc1265 will delay entering burst mode operation and efficiency will be degraded at low currents. inductor core selection with the value of l selected, the type of inductor must be chosen. basically, there are two kinds of losses in an inductor; core and copper losses. core losses are dependent on the peak-to-peak ripple current and core material. however it is independent of the physical size of the core. by increasing the induc- tance, the peak-to-peak inductor ripple current will de- crease, therefore reducing core loss. utilizing low core loss material, such as molypermalloy or kool m m ? will allow user to concentrate on reducing copper loss and preventing saturation. although higher inductance reduces core loss, it in- creases copper loss as it requires more windings. when space is not at a premium, larger wire can be used to reduce the wire resistance. this also prevents excessive heat dissipation. catch diode selection losses in the catch diode depend on forward drop and switching times. therefore schottky diodes are a good choice for low drop and fast switching times. the catch diode carries load current during the off time. the average diode current is therefore dependent on the applicatio s i for atio wu uu
8 ltc1265/ltc1265-3.3/LTC1265-5 p-channel switch duty cycle. at high input voltages, the diode conducts most of the time. as v in approaches v out , the diode conducts only a small fraction of the time. the most stressful condition for the diode is when the output is short circuited. under this condition, the diode must safely handle i sc(pk) at close to 100% duty cycle. most ltc1265 circuits will be well served by either a 1n5818 or a mbrs130lt3 schottky diode. an mbrs0520 is a good choice for i out(max) 500ma. c in in continuous mode, the input current of the converter is a square wave of duty cycle v out /v in . to prevent large voltage transients, a low esr input capacitor must be used. in addition, the capacitor must handle a high rms current. the c in rms current is given by: i rms ? i out [v out (v in v out )] 1 / 2 v in (a rms ) this formula has a maximum at v in = 2v out , where i rms = i out /2. this simple worst case is commonly used for design because even significant deviations do not offer much relief. note that capacitor manufacturers ripple current ratings are often based on only 2000 hours life- time. this makes it advisable to further derate the capaci- tor, or to choose a capacitor rated at a higher temperature than required. do not underspecify this component . an additional 0.1 m f ceramic capacitor is also required on pwr v in for high frequency decoupling. c out the selection of c out is based upon the effective series resistance (esr) for proper operation of the ltc1265. the required esr of c out is: esr cout < 50mv/i ripple where i ripple is the ripple current of the inductor. for the case where the i ripple is 25mv/r sense , the required esr of c out is: esr cout < 2(r sense ) to avoid overheating, the output capacitor must be sized to handle the ripple current generated by the inductor. the worst-case rms ripple current in the output capacitor is given by: i rms ? 150mv 2(r sense ) (a rms ) generally, once the esr requirement for c out has been met, the rms current rating far exceeds the i ripple(p-p) requirement. esr is a direct function of the volume of the capacitor. manufacturers such as nichicon, avx and sprague should be considered for high performance capacitors. the os-con semiconductor dielectric capacitor available from sanyo has the lowest esr for its size at a somewhat higher price. in surface mount applications, multiple capacitors may have to be paralleled to meet the capacitance, esr or rms current handling requirement of the application. alumi- num electrolyte and dry tantalum capacitors are both available in surface mount configurations. in the case of tantalum, it is critical that the capacitors are both available in surface mount configuration and are surge tested for use in switching power supplies. an excellent choice is the avx tps series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. consult the manufacturer for other specific recommendations. when the capacitance of c out is made too small, the output ripple at low frequencies will be large enough to trip the voltage comparator. this causes burst mode opera- tion to be activated when the ltc1265 would normally be in continuous operation. the effect will be most pro- nounced with low value of r sense and can be improved at higher frequencies with lower values of l. low-battery detection the low-battery comparator senses the input voltage through an external resistive divider. this divided voltage connects to the (C) input of a voltage comparator (pin 4) which is compared with a 1.25v reference voltage. ne- glecting pin 4 bias current, the following expression is used for setting the trip limit: v lb_trip = 1.25 1 + r4 r3 ) ) applicatio s i for atio wu uu
9 ltc1265/ltc1265-3.3/LTC1265-5 figure 3. low-battery comparator figure 4. ltc1265 adjustable configuration the output, pin 3, is an n-channel open drain that goes low when the battery voltage is below the threshold set by r3 and r4. in shutdown, the comparator is disabled and pin 3 is in a high impedance state. ltc1265 adjustable applications the ltc1265 develops a 1.25v reference voltage between the feedback (pin 9) terminal and signal ground (see figure 4). by selecting resistor r1, a constant current is caused to flow through r1 and r2 to set overall output voltage. the regulated output voltage is determined by: v out = 1.25 1 + r2 r1 ) ) for most applications a 30k resistor is suggested for r1. to prevent stray pickup, a 100pf capacitor is suggested across r1 located close to the ltc1265. absolute maximum ratings and latchup prevention the absolute maximum ratings specify that sw (pin 14) can never exceed v in (pins 1, 2, 13) by more than 0.3v. normally this situation should never occur. it could, however, if the output is held up while the v in supply is pulled down. a condition where this could potentially occur is when a battery is supplying power to an ltc1265 regulator and also to one or more loads in parallel with the the regulators v in . if the battery is disconnected while the ltc1265 regulator is supplying a light load and one of the parallel circuits has a heavy load, the input capacitor of the ltc1265 regulator could be pulled down faster than the output capacitor, causing the absolute maximum ratings to be exceeded. the result is often a latchup which can be destructive if v in is reapplied quickly. battery disconnect is possible as a result of mechanical stress, bad battery contacts or use of a lithium-ion battery with a built-in internal disconnect. the user needs to assess his/her application to determine whether this situation could occur. if so, additional protection is necessary. prevention against latchup can be accomplished by simply connecting a schottky diode across the sw and v in pins as shown in figure 5. the diode will normally be reverse biased unless v in is pulled below v out at which time the diode will clamp the (v out C v in ) potential to less than the 0.6v required for latchup. note that a low leakage schottky should be used to minimize the effect on no- load supply current. schottky diodes such as mbr0530, bas85 and bat84 work well. another more serious effect of the protection diode leakage is that at no load with nothing to provide a sink for this leakage current, the 1.25v reference ltc1265 v in 4 3 + ltc1265 f03 r4 r3 100pf r1 r2 v out 9 ltc1265 f04 v fb ltc1265 11 sgnd 1265 f05 pwr v in v out latchup protection schottky sw ltc1265 + figure 5. preventing absolute maximum ratings from being exceeded applicatio s i for atio wu uu
10 ltc1265/ltc1265-3.3/LTC1265-5 applicatio n s i n for m atio n wu u u output voltage can potentially float above the maximum allowable tolerance. to prevent this from occuring, a resistor must be connected between v out and ground with a value low enough to sink the maximum possible leakage current. thermal considerations in a majority of applications, the ltc1265 does not dissipate much heat due to its high efficiency. however, in applications where the switching regulator is running at high duty cycles or the part is in dropout with the switch turned on continuously (dc), the user will need to do some thermal analysis. the goal of the thermal analysis is to determine whether the power dissipated by the regulator exceeds the maximum junction temperature of the part. the temperature rise is given by: t r = p( q ja ) where p is the power dissipated by the regulator and q ja is the thermal resistance from the junction of the die to the ambient temperature. the junction temperature is simply given by: t j = t r + t a as an example, consider the ltc1265 is in dropout at an input voltage of 4v with a load current of 0.5a. from the typical performance characteristics graph of switch re- sistance, the on resistance of the p- channel is 0.55 w . therefore power dissipated by the part is: p = i 2 (r dson ) = 0.1375w for the so package, the q ja is 110 c/w. therefore the junction temperature of the regulator when it is operating in ambient temperature of 25 c is: t j = 0.1375(110) + 25 = 40.1 c remembering that the above junction temperature is obtained from a r dson at 25 c, we need to recalculate the junction temperature based on a higher r dson since it increases with temperature. however, we can safely as- sume that the actual junction temperature will not exceed the absolute maximum junction temperature of 125 c. now consider the case of a 1a regulator with v in = 4v and t a = 65 c. starting with the same 0.55 w assumption for r dson , the t j calculation will yield 125 c. but from the graph, this will increase the r dson to 0.76 w , which when used in the above calculation yields an actual t j > 148 c. therefore the ltc1265 would be unsuitable for a 4v input, 1a output regulator operating at t a = 65 c. board layout checklist when laying out the printed circuit board, the following checklist should be used to ensure proper operation of the ltc1265. these items are also illustrated graphically in the layout diagram of figure 6. check the following in your layout: 1. are the signal and power grounds segregated? the ltc1265 signal ground (pin 11) must return to the (C) plate of c out . the power ground (pin 12) returns to the anode of the schottky diode, and the (C) plate of c in , whose leads should be as short as possible. 2. does the (+) plate of the c in connect to the power v in (pins 1,13) as close as possible? this capacitor pro- vides the ac current to the internal p-channel mosfet and its driver. 3. is the input decoupling capacitor (0.1 m f) connected closely between power v in (pins 1,13) and power ground (pin 12)? this capacitor carries the high fre- quency peak currents. 4. is the schottky diode closely connected between the power ground (pin 12) and switch (pin 14)? 5. does the ltc1265 sense C (pin 7) connect to a point close to r sense and the (+) plate of c out ? in adjustable applications, the resistive divider, r1 and r2, must be connected between the (+) plate of c out and signal ground. 6. are the sense C and sense + leads routed together with minimum pc trace spacing? the 1000pf capacitor between pins 7 and 8 should be as close as possible to the ltc1265. 7. is shdn (pin 10) actively pulled to ground during normal operation? the shdn pin is high impedance and must not be allowed to float.
11 ltc1265/ltc1265-3.3/LTC1265-5 figure 7. c t waveforms troubleshooting hints since efficiency is critical to ltc1265 applications, it is very important to verify that the circuit is functioning correctly in both continuous and burst mode operation. as the ltc1265 is highly tolerant of poor layout, the output voltage will still be regulated. therefore, monitoring the output voltage will not tell you whether you have a good or bad layout. the waveform to monitor is the voltage on the timing capacitor pin 5. in continuous mode the voltage on the c t pin is a sawtooth with approximately 0.9v p-p swing. this voltage should never dip below 2v as shown in figure 7a. when the load currents are low (i load < i burst ) burst mode operation occurs. the voltage on c t pin now falls to ground for periods of time as shown in figure 7b. during this time the ltc1265 is in sleep mode with quiescent current reduced to 160 m a. the inductor current should also be monitored. if the circuit is poorly decoupled, the peak inductor current will be haphazard as in figure 8a. a well decoupled ltc1265 has a clean inductor current as in figure 8b. figure 6. ltc1265 layout diagram (see board layout checklist) v in v in lb out lb in c t i th sense 1000pf 1000pf ltc1265 sense + n/c (v fb ) shdn sgnd pgnd pwr v in sw 14 13 12 11 10 9 8 c in d1 0.1 m f c out r sense l v out output divider required with adjustable version only pwr v in 1k 3900pf 1 2 3 4 5 6 7 ltc1265 f06 shdn bold lines indicate high path currents r1 r2 + + 3.3v 2.4v 0v time ltc1265 f07a (a) continuous mode operation voltage at c t (pin 5) 3.3v 2.4v 0v time ltc1265 f07b (b) burst mode operation voltage at c t (pin 5) sleep mode applicatio s i for atio wu uu
12 ltc1265/ltc1265-3.3/LTC1265-5 22 m h 0.125 w v out 3.3v 0.8a c out 1000pf 100pf 3900pf 1k 0.1 m f c in d1 pwr v in v in sw pgnd sense + sense sgnd i th c t ltc1265 f09 shdn ltc1265-3.3 v in 5v + + figure 9. design example circuit figure 10. design example efficiency curve load current (ma) 0.01 70 efficiency (% ) 75 80 85 90 100 0.1 1.0 1265 g11 95 l = dale lpt4545-220 (22 m h) v out = 3.3v c t = 100pf design example as a design example, assume v in = 5v, v out = 3.3v, i max = 0.8a and f = 250khz. with this information we can easily calculate all the important components. from (1), r sense = 100mv/0.8 = 0.125 w from (2) and assuming v d = 0.4v, c t @ 100pf using (3), the value of the inductor is: l 3 5.2(10 5 )(0.125)(100pf)3.3v = 22 m h for the catch diode, a mbrs130lt3 or 1n5818 will be sufficient in this application. c in will require an rms current rating of at least 0.4a at temperature, and c out will require an esr of (from 5): esr cout < 0.25 w the inductor ripple current is given by: i ripple = v out + v d l ) ) t off = 0.22a at light loads the peak inductor current is at: i peak = 25mv/0.125 = 0.2a therefore, at load current less than 0.1a the ltc1265 will be in burst mode operation. figure 9 shows the complete circuit and figure 10 shows the efficiency curve with the above calculated component values. (a) poorly decoupled ltc1265 (b) well decoupled ltc1265 figure 8. inductor waveforms applicatio s i for atio wu uu
13 ltc1265/ltc1265-3.3/LTC1265-5 high efficiency 5v to 3.3v converter positive-to-negative (C5v) converter l1 ? 47 m h 1k v in 5v pwr v in v in ltc1265-3.3 sw pgnd sgnd shdn shdn 1000pf nc sense + 4 3 5 6 7 14 2 1, 13 12 mbrs130lt1 11 10 9 8 lb in lb out c t i thr sense ltc1265 ta02 avx tpsd107k010 avx tpse227k010 coilcraft d03316-473 dale wsl2010-0.1-1% * ** ? ?? 3900pf 270pf 0.1 m f c in * 100 m f 10v r sense ?? 0.1 w c out ** 220 m f 10v v out 3.3v 1a + + 1k v in 3.5v to 7.5v pwr v in v in LTC1265-5 sw pgnd sgnd shdn shdn 1000pf sense + 4 tp0610l 3 5 6 7 14 d1 100k 2 1, 13 12 11 10 8 lb in lb out c t i thr sense ltc1265 ta03 avx tpsd226k025 avx tpsd107k010 l1 selection * ** ? 2200pf 220pf 0.1 m f c in * 22 m f 25v 2 r sense ?? 0.1 w c out ** 100 m f 10v v out ?v l1 ? 50 m h manufacturer coilcraft coiltronics dale sumida v in (v) 3.5 4.0 5.0 6.0 7.0 7.5 i out(max) (ma) 360 430 540 630 720 740 part no. do3316-473 ctx50-4 lpt4545-500la cd74-470 ?? d1 irc lrc2010-01-r100-j = mbrs130lt3 + + typical applicatio s u
14 ltc1265/ltc1265-3.3/LTC1265-5 9v to 12v and C 12v outputs 5v buck-boost converter l1a ?? 33 m h l1b ?? 33 m h 1k v in 3.5v to 7.5v pwr v in v in ltc1265 sw pgnd sgnd shdn shdn 0.01 m f 100pf v fb sense + 4 3 5 6 7 14 2 1, 13 12 1n5818 11 10 9 8 lb in lb out c t i thr sense ltc1265 f09 3300pf 75pf 0.1 m f c in * 100 m f 16v c out * 100 m f 10v r sense ** 0.162 w v out 5v 33 m f 10v* 1 2 75k 25k 4 3 top view 3 1 2 4 l1b l1b l1a l1a manufacturer coiltronics dale part no. ctx33-4 lpt4545-330la v in (v) 3.5 4.0 5.0 6.0 7.0 7.5 i out(max) (ma) 240 275 365 490 610 665 sanyo os-con capacitor irc lrc2010-01-r162-j l1a, l2a selection * ** ? + + + l1a ?? 50 m h l1b ?? 50 m h 1k v in 4v to 12v pwr v in v in ltc1265 sw pgnd sgnd shdn shdn 0.01 m f 100pf v fb sense + 4 3 5 6 7 14 2 1, 13 12 mbrs130lt3 mbrs130lt3 11 10 9 8 lb in lb out c t i thr sense ltc1265 ta05 3300pf 75pf 0.1 m f c in * 68 m f 20v c out * 68 m f 20v c out * 68 m f 20v r sense * 0.162 w v out ?2v v out 12v 33 m f** 25v 1 2 301k 34k 4 3 top view 3 1 2 4 l1b l1b l1a l1a manufacturer coiltronics dale part no. ctx50-4 lpt4545-500la v in (v) 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 i out(max) (ma) 40 60 80 100 115 130 150 165 180 si19430dy avx tpse686k020 avx tpse336k025 irc lrc2010-01-r162-j l1a,l2a selection * ** ? ?? 1n914 + + + + typical applicatio s u
15 ltc1265/ltc1265-3.3/LTC1265-5 2.5mm max height 5v-to-3.3v (500ma) logic selectable 0v/3.3v/5v 700ma regulator 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 represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. l1 ?? 18 m h v in 3.5v to 12.5v pwr v in v in ltc1265-3.3 sw pgnd sgnd shdn shdn 1000pf 3300pf n/c sense + 4 3 5 6 7 14 2 1, 13 12 mbrs0520lt1 11 10 9 8 lb in lb out c t i thr sense ltc1265 ta06 avx tajb156k010 avx tajb226k06 irc lrc2010-01-r200-j sumida cls62-180 * ** ? ?? 51pf 0.1 m f c in * 15 m f 10v 2 c out ** 22 m f 6.3v 2 v out 3.3v 500ma r sense ? 0.20 w 1k + + l1 ?? 33 m h 45.3k 75k v in 3.5v to 12.5v pwr v in v in ltc1265 sw pgnd sgnd shdn v shdn ??? 1000pf 3300pf 100pf v fb sense + 4 3 5 6 7 14 21, 13 12 mbrs130lt3 11 10 9 8 lb in lb out c t i thr sense ltc1265 ta07 dale 593d68x0020e2w dale 593d107x0010d2w irc lrc2010-01-r15-j l1 selection * ** ? ?? 75pf 0.1 m f c in * 68 m f 20v c out ** 100 m f 10v v out 0v/3.3v/5v 700ma r sense ? 0.15 w 0v: v out = 5v 5v: v out = 3.3v 56.2k manufacturer coilcraft coiltronics dale sumida part no. do3316-333 ctx33-4 lpt4545-330la cd74-330 ??? v shdn = 0v: v out = 3.3v/5v = 5v: v out = 0v 1k + + typical applicatio s u
16 ltc1265/ltc1265-3.3/LTC1265-5 4-nicad battery charger ? linear technology corporation 1995 126535fb lt/lcg 0800 2k rev b ? printed in usa dimension in inches (millimeters) unless otherwise noted. part number description comments ltc1143 dual step-down switching regulator controller dual version of ltc1147 ltc1147 step-down switching regulator controller nonsynchronous, 8-pin, v in 16v ltc1148hv step-down switching regulator controller synchronous, v in 20v ltc1174 step-down switching regulator with internal 0.5a switch v in 18.5v, comparator/low battery detector ltc1474/ltc1475 low quiescent current step-down regulators monolithic, i q = 40 m a, 400ma, ms8 ltc1574 step-down switching regulator with internal 0.5a switch v in 18.5v, comparator and schottky diode ltc1622 low input voltage step-down dc/dc controller constant frequency, 2v to 10v v in , ms8 ltc1627 monolithic synchronous step-down switching regulator constant frequency, i out to 500ma, 2.65v to 8.5v v in ltc1772 constant frequency step-down dc/dc controller sot-23, 2.2v to 9.8v v in l1 ?? 100 m h 30k 138k v in 8v to 12.5v pwr v in v in ltc1265 sw pgnd sgnd shdn 1000pf 3300pf 100pf v fb sense + 4 3 5 6 7 14 2 1, 13 12 mbrs130lt3 mbrs130lt3 11 10 9 8 lb in lb out c t i thr sense ltc1265 ta08 dale 593d226x0025d2w dale 593d107x0016e2w dale wsl2010-0.10-1% l1 selection * ** ? ?? 270pf 0.1 m f c in * 22 m f, 25v c out ** 100 m f 10v v out 4 nicad 1a fast charge 0.1a trickle charge r sense ? 0.10 w 1k manufacturer coilcraft coiltronics sumida part no. do3316-104 ctx100-4p cd105-101 charger on/off vn2222l 51 w fast charge: = 0v trickle charge: > 2v + + 0.016 ?0.050 (0.406 ?1.270) 0.010 ?0.020 (0.254 ?0.508) 45 0 ?8 typ 0.008 ?0.010 (0.203 ?0.254) s14 1298 0.053 ?0.069 (1.346 ?1.752) 0.014 ?0.019 (0.355 ?0.483) typ 0.004 ?0.010 (0.101 ?0.254) 0.050 (1.270) bsc 1 2 3 4 0.150 ?0.157** (3.810 ?3.988) 14 13 0.337 ?0.344* (8.560 ?8.738) 0.228 ?0.244 (5.791 ?6.197) 12 11 10 9 5 6 7 8 dimension does not include mold flash. mold flash shall not exceed 0.006" (0.152mm) per side dimension does not include interlead flash. interlead flash shall not exceed 0.010" (0.254mm) per side * ** linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear-tech.com typical applicatio s u u package descriptio related parts s package 14-lead plastic small outline (narrow 0.150) (ltc dwg # 05-08-1610)

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