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1 lt1308a/LT1308B , ltc and lt are registered trademarks of linear technology corporation. high current, micropower single cell, 600khz dc/dc converters n 5v at 1a from a single li-ion cell n 5v at 800ma in sepic mode from four nicd cells n fixed frequency operation: 600khz n boost converter outputs up to 34v n thin 1.1mm height tssop packaging available n starts into heavy loads n automatic burst mode tm operation at light load (lt1308a) n continuous switching at light loads (LT1308B) n low v cesat switch: 300mv at 2a n pin-for-pin upgrade compatible with lt1308 n lower quiescent current in shutdown: 1 m a (max) n improved accuracy low-battery detector reference: 200mv 2% n gsm/cdma phones n digital cameras n lcd bias supplies n answer-back pagers n gps receivers n battery backup supplies n handheld computers the lt ? 1308a/LT1308B are micropower, fixed frequency step-up dc/dc converters that operate over a 1v to 10v input voltage range. they are improved versions of the lt1308 and are recommended for use in new designs. the lt1308a features automatic shifting to power saving burst mode operation at light loads and consumes just 140 m a at no load. the LT1308B features continuous switching at light loads and operates at a quiescent current of 2.5ma. both devices consume less than 1 m a in shutdown. low-battery detector accuracy is significantly tighter than the lt1308. the 200mv reference is specified at 2% at room and 3% over temperature. the shutdown pin enables the device when it is tied to a 1v or higher source and does not need to be tied to v in as on the lt1308. an internal v c clamp results in improved transient response and the switch voltage rating has been increased to 36v, enabling higher output voltage applications. the lt1308a/LT1308B are available in the 8-lead so and the 14-lead tssop packages. burst mode is a trademark of linear technology corporation. figure 1. LT1308B single li-ion cell to 5v/1a dc/dc converter converter efficiency v in sw fb LT1308B l1 4.7 h d1 lbo lbi 47k r2 100k r1* 309k 5v 1a 100pf 1308a/b f01a c1 47 f c2 220 f li-ion cell v c gnd shdn shutdown c1: avx tajc476m010 c2: avx tpsd227m006 d1: ir 10bq015 + + l1: murata lqh6c4r7 *r1: 887k for v out = 12v load current (ma) 1 efficiency (%) 95 90 85 80 75 70 65 60 55 50 10 100 1000 1308a/b f01b v in = 4.2v v in = 1.5v v in = 2.5v v in = 3.6v applicatio s u features typical applicatio u descriptio u
2 lt1308a/LT1308B a u g w a w u w a r b s o lu t exi t i s order part number wu u package / o rder i for atio e lectr ic al c c hara terist ics (note 1) v in , shdn, lbo voltage ......................................... 10v sw voltage ............................................... C 0.4v to 36v fb voltage ....................................................... v in + 1v v c voltage ................................................................ 2v lbi voltage ................................................. C 0.1v to 1v current into fb pin .............................................. 1ma s8 part marking lt1308acs8 lt1308ais8 LT1308Bcs8 LT1308Bis8 the l denotes specifications which apply over the full operating temperature range, otherwise specifications are t a = 25 c. commercial grade 0 c to 70 c. v in = 1.1v, v shdn = v in , unless otherwise noted. symbol parameter conditions min typ max units i q quiescent current not switching, lt1308a 140 240 m a switching, LT1308B 2.5 4 ma v shdn = 0v (lt1308a/LT1308B) 0.01 1 m a v fb feedback voltage l 1.20 1.22 1.24 v i b fb pin bias current (note 3) l 27 80 na reference line regulation 1.1v v in 2v l 0.03 0.4 %/v 2v v in 10v 0.01 0.2 %/v minimum input voltage 0.92 1 v g m error amp transconductance d i = 5 m a60 m mhos a v error amp voltage gain 100 v/v f osc switching frequency v in = 1.2v l 500 600 700 khz maximum duty cycle l 82 90 % switch current limit duty cyle = 30% (note 4) 2 3 4.5 a switch v cesat i sw = 2a (25 c, 0 c), v in = 1.5v 290 350 mv i sw = 2a (70 c), v in = 1.5v 330 400 mv burst mode operation switch current limit v in = 2.5v, circuit of figure 1 400 ma (lt1308a) t jmax = 125 c, q ja = 190 c/w 1308b 1308bi 1308a 1308ai consult factory for military grade parts. operating temperature range commercial ............................................ 0 c to 70 c extended commerial (note 2) ........... C 40 c to 85 c industrial ........................................... C 40 c to 85 c storage temperature range ................ C 65 c to 150 c lead temperature (soldering, 10 sec)................. 300 c order part number lt1308acf LT1308Bcf t jmax = 125 c, q ja = 80 c/w f package 14-lead plastic tssop 1 2 3 4 5 6 7 top view 14 13 12 11 10 9 8 v c fb shdn gnd gnd gnd gnd lbo lbi v in v in sw sw sw (note 6) 1 2 3 4 8 7 6 5 top view lbo lbi v in sw v c fb shdn gnd s8 package 8-lead plastic so
3 lt1308a/LT1308B e lectr ic al c c hara terist ics the l denotes specifications which apply over the full operating temperature range, otherwise specifications are t a = 25 c. industrial grade C 40 c to 85 c. v in = 1.2v, v shdn = v in , unless otherwise noted. symbol parameter conditions min typ max units i q quiescent current not switching, lt1308a l 140 240 m a switching, LT1308B l 2.5 4 ma v shdn = 0v (lt1308a/LT1308B) l 0.01 1 m a v fb feedback voltage l 1.19 1.22 1.25 v i b fb pin bias current (note 3) l 27 80 na reference line regulation 1.1v v in 2v l 0.05 0.4 %/v 2v v in 10v l 0.01 0.2 %/v minimum input voltage 0.92 1 v g m error amp transconductance d i = 5 m a60 m mhos a v error amp voltage gain 100 v/v f osc switching frequency l 500 600 750 khz maximum duty cycle l 82 90 % switch current limit duty cyle = 30% (note 4) 2 3 4.5 a switch v cesat i sw = 2a (25 c, C 40 c), v in = 1.5v 290 350 mv i sw = 2a (85 c), v in = 1.5v 330 400 mv burst mode operation switch current limit v in = 2.5v, circuit of figure 1 400 ma (lt1308a) shutdown pin current v shdn = 1.1v l 2 5 m a v shdn = 6v l 20 35 m a v shdn = 0v 0.01 0.1 m a lbi threshold voltage 196 200 204 mv l 193 200 207 mv lbo output low i sink = 50 m a l 0.1 0.25 v lbo leakage current v lbi = 250mv, v lbo = 5v l 0.01 0.1 m a lbi input bias current (note 5) v lbi = 150mv 33 100 na low-battery detector gain 3000 v/v switch leakage current v sw = 5v l 0.01 10 m a the l denotes specifications which apply over the full operating temperature range, otherwise specifications are t a = 25 c. commercial grade 0 c to 70 c. v in = 1.1v, v shdn = v in , unless otherwise noted. symbol parameter conditions min typ max units shutdown pin current v shdn = 1.1v l 25 m a v shdn = 6v l 20 35 m a v shdn = 0v l 0.01 0.1 m a lbi threshold voltage 196 200 204 mv l 194 200 206 mv lbo output low i sink = 50 m a l 0.1 0.25 v lbo leakage current v lbi = 250mv, v lbo = 5v l 0.01 0.1 m a lbi input bias current (note 5) v lbi = 150mv 33 100 na low-battery detector gain 3000 v/v switch leakage current v sw = 5v l 0.01 10 m a
4 lt1308a/LT1308B typical perfor m a n ce characteristics uw LT1308B 3.3v output efficiency lt1308a 5v output efficiency lt1308a 3.3v output efficiency LT1308B 12v output efficiency switch saturation voltage vs current switch current limit vs duty cycle note 4: switch current limit guaranteed by design and/or correlation to static tests. duty cycle affects current limit due to ramp generator (see block diagram). note 5: bias current flows out of lbi pin. note 6: connect the four gnd pins (pins 4C7) together at the device. similarly, connect the three sw pins (pins 8C10) together and the two v in pins (pins 11, 12) together at the device. note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: the lt1308acs8, lt1308acf, LT1308Bcs8 and LT1308Bcf are designed, characterized and expected to meet the industrial temperature limits, but are not tested at C 40 c and 85 c. i grade devices are guaranteed over the C40 c to 85 c operating temperature range. note 3: bias current flows into fb pin. e lectr ic al c c hara terist ics load current (ma) 95 90 85 80 75 70 65 60 55 50 1 100 1000 1308a/b g01 10 efficiency (%) v in = 1.8v v in = 2.5v v in = 1.2v load current (ma) 95 90 85 80 75 70 65 60 55 50 1 100 1000 1308a/b g02 10 efficiency (%) v in = 1.8v v in = 2.5v v in = 1.2v load current (ma) 1 efficiency (%) 95 90 85 80 75 70 65 60 55 50 10 100 1000 1308a/b g03 v in = 4.2v v in = 2.5v v in = 3.6v v in = 1.5v load current (ma) 90 85 80 75 70 65 60 55 50 1 100 1000 1308a/b g04 10 efficiency (%) v in = 5v v in = 3.3v switch current (a) 0 switch v cesat (mv) 2.0 85 c 1308 g06 0.5 1.0 1.5 500 400 300 200 100 0 25 c ?0 c duty cycle (%) 0 current limit (a) 3.0 3.5 80 1308 ?g05 2.5 2.0 20 40 60 100 4.0
5 lt1308a/LT1308B typical perfor m a n ce characteristics uw shdn pin voltage (v) 0 shdn pin current ( a) 50 40 30 20 10 0 8 1308 g07 2 4 6 10 ?0 c 25 c 85 c temperature ( c) ?0 ?5 bias current (na) 050 25 75 100 1308 ?g08 80 70 60 50 40 30 20 10 0 lbi fb temperature ( c) ?0 ?5 v ref (mv) 050 25 75 100 1308 ?g09 203 202 201 200 199 198 197 196 195 temperature ( c) ?0 ?.5 frequency (khz) 050 25 75 100 1308 ?g10 800 750 700 650 600 550 500 450 400 temperature ( c) ?0 ?5 quiescent current ( a) 050 25 75 100 1308 ?g11 180 170 160 150 140 130 120 110 100 temperature ( c) ?0 ?5 v fb (v) 050 25 75 100 1308 ?g12 1.25 1.24 1.23 1.22 1.21 1.20 1.19 1.18 shdn pin bias current vs voltage fb, lbi bias current vs temperature low battery detector reference vs temperature oscillator frequency vs temperature lt1308a quiescent current vs temperature feedback pin voltage vs temperature
6 lt1308a/LT1308B v c (pin 1): compensation pin for error amplifier. con- nect a series rc from this pin to ground. typical values are 47k w and 100pf. minimize trace area at v c . fb (pin 2): feedback pin. reference voltage is 1.22v. connect resistive divider tap here. minimize trace area at fb. set v out according to: v out = 1.22v(1 + r1/r2). shdn (pin 3): shutdown. ground this pin to turn off switcher. to enable, tie to 1v or more. shdn does not need to be at v in to enable the device. gnd (pin 4): ground. connect directly to local ground plane. ground plane should enclose all components associated with the lt1308. pcb copper connected to pin 4 also functions as a heat sink. maximize this area to keep chip heating to a minimum. sw (pin 5): switch pin. connect inductor/diode here. minimize trace area at this pin to keep emi down. v in (pin 6): supply pin. must have local bypass capacitor right at the pin, connected directly to ground. lbi (pin 7): low-battery detector input. 200mv refer- ence. voltage on lbi must stay between C100mv and 1v. low-battery detector does not function with shdn pin grounded. float lbi pin if not used. lbo (pin 8): low-battery detector output. open collec- tor, can sink 50 m a. a 220k w pull-up is recommended. lbo is high impedance when shdn is grounded. pi n fu n ctio n s uuu so-8 package tssop package v c (pin 1): compensation pin for error amplifier. con- nect a series rc from this pin to ground. typical values are 47k w and 100pf. minimize trace area at v c . fb (pin 2): feedback pin. reference voltage is 1.22v. connect resistive divider tap here. minimize trace area at fb. set v out according to: v out = 1.22v(1 + r1/r2). shdn (pin 3): shutdown. ground this pin to turn off switcher. to enable, tie to 1v or more. shdn does not need to be at v in to enable the device. gnd (pins 4, 5, 6, 7): ground. connect directly to local ground plane. ground plane should enclose all compo- nents associated with the lt1308. pcb copper con- nected to these pins also functions as a heat sink. connect all pins to ground copper to get the best heat transfer. this keeps chip heating to a minimum. sw (pins 8, 9, 10): switch pins. connect inductor/diode here. minimize trace area at these pins to keep emi down. connect all sw pins together at the package. v in (pins 11, 12): supply pins. must have local bypass capacitor right at the pins, connected directly to ground. connect both v in pins together at the package. lbi (pin 13): low-battery detector input. 200mv refer- ence. voltage on lbi must stay between C100mv and 1v. low-battery detector does not function with shdn pin grounded. float lbi pin if not used. lbo (pin 14): low-battery detector output. open collec- tor, can sink 50 m a. a 220k w pull-up is recommended. lbo is high impedance when shdn is grounded.
7 lt1308a/LT1308B figure 2a. lt1308a/LT1308B block diagram (so-8 package) block diagra s w + + + + + + + s comparator ramp generator r bias v c 2v be g m q2 10 q1 fb fb enable *hysteresis in lt1308a only 200mv a = 3 ff a2 a1 q4 * error amplifier a4 0.03 driver sw gnd 1308 bd2a q3 q s 600khz oscillator 5 lbo lbi shdn shutdown 3 7 1 4 r6 40k r5 40k r1 (external) r3 30k r4 140k 2 v in v in v in v out 6 8 r2 (external) + + + + + + + s comparator ramp generator r bias v c 2v be g m q2 10 q1 fb fb enable *hysteresis in lt1308a only 200mv a = 3 ff a2 a1 q4 * error amplifier a4 0.03 driver sw gnd 1308 bd2b q3 q s 600khz oscillator 8 sw 9 sw lbo lbi shdn shutdown 3 13 1 4 gnd 5 gnd 6 gnd 7 r6 40k r5 40k r1 (external) r3 30k r4 140k 2 v in v in v in v in v out 11 12 14 r2 (external) 10 figure 2b. lt1308a/LT1308B block diagram (tssop package)
8 lt1308a/LT1308B trace a: lt1308a v out , 100mv/div ac coupled 800ma i load 50ma trace b: LT1308B v out , 100mv/div ac coupled v in = 3v 200 m s/div 1308 f03 (circuit of figure 1) figure 3. lt1308a exhibits burst mode operation output voltage ripple at 50ma load, LT1308B does not operation the lt1308a combines a current mode, fixed frequency pwm architecture with burst mode micropower operation to maintain high efficiency at light loads. operation can be best understood by referring to the block diagram in figure 2. q1 and q2 form a bandgap reference core whose loop is closed around the output of the converter. when v in is 1v, the feedback voltage of 1.22v, along with an 80mv drop across r5 and r6, forward biases q1 and q2s base collector junctions to 300mv. because this is not enough to saturate either transistor, fb can be at a higher voltage than v in . when there is no load, fb rises slightly above 1.22v, causing v c (the error amplifiers output) to decrease. when v c reaches the bias voltage on hysteretic comparator a1, a1s output goes low, turning off all circuitry except the input stage, error amplifier and low- battery detector. total current consumption in this state is 140 m a. as output loading causes the fb voltage to decrease, a1s output goes high, enabling the rest of the ic. switch current is limited to approximately 400ma initially after a1s output goes high. if the load is light, the output voltage (and fb voltage) will increase until a1s output goes low, turning off the rest of the lt1308a. low frequency ripple voltage appears at the output. the ripple frequency is dependent on load current and output capaci- tance. this burst mode operation keeps the output regu- lated and reduces average current into the ic, resulting in high efficiency even at load currents of 1ma or less. if the output load increases sufficiently, a1s output remains high, resulting in continuous operation. when the lt1308a is running continuously, peak switch current is controlled by v c to regulate the output voltage. the switch is turned on at the beginning of each switch cycle. when the summation of a signal representing switch current and a ramp generator (introduced to avoid subharmonic oscil- lations at duty factors greater than 50%) exceeds the v c signal, comparator a2 changes state, resetting the flip-flop and turning off the switch. output voltage increases as switch current is increased. the output, attenuated by a resistor divider, appears at the fb pin, closing the overall loop. frequency compensation is provided by an external series rc network connected between the v c pin and ground. applicatio n s i n for m atio n wu u u low-battery detector a4s open-collector output (lbo) pulls low when the lbi pin voltage drops below 200mv. there is no hysteresis in a4, allowing it to be used as an amplifier in some applications. the entire device is dis- abled when the shdn pin is brought low. to enable the converter, shdn must be at 1v or greater. it need not be tied to v in as on the lt1308. the LT1308B differs from the lt1308a in that there is no hysteresis in comparator a1. also, the bias point on a1 is set lower than on the LT1308B so that switching can occur at inductor current less than 100ma. because a1 has no hysteresis, there is no burst mode operation at light loads and the device continues switching at constant frequency. this results in the absence of low frequency output voltage ripple at the expense of efficiency. the difference between the two devices is clearly illus- trated in figure 3. the top two traces in figure 3 shows an lt1308a/LT1308B circuit, using the components indi- cated in figure 1, set to a 5v output. input voltage is 3v. load current is stepped from 50ma to 800ma for both circuits. low frequency burst mode operation voltage ripple is observed on trace a, while none is observed on trace b. at light loads, the LT1308B will begin to skip alternate cycles. the load point at which this occurs can be de- creased by increasing the inductor value. however, output ripple will continue to be significantly less than the lt1308a output ripple. further, the LT1308B can be forced into micropower mode, where i q falls from 3ma to 200 m a by sinking 40 m a or more out of the v c pin. this stops switching by causing a1s output to go low.
9 lt1308a/LT1308B applicatio n s i n for m atio n wu u u waveforms for a LT1308B 5v to 12v boost converter using a 10 m f ceramic output capacitor are pictured in figures 4 and 5. in figure 4, the converter is operating in continuous mode, delivering a load current of approxi- mately 500ma. the top trace is the output. the voltage increases as inductor current is dumped into the output capacitor during the switch off time, and the voltage decreases when the switch is on. ripple voltage is in this case due to capacitance, as the ceramic capacitor has little esr. the middle trace is the switch voltage. this voltage alternates between a v cesat and v out plus the diode drop. the lower trace is the switch current. at the beginning of the switch cycle, the current is 1.2a. at the end of the switch on time, the current has increased to 2a, at which point the switch turns off and the inductor current flows into the output capacitor through the diode. figure 5 depicts converter waveforms at a light load. here the converter operates in discontinuous mode. the inductor current reaches zero during the switch off time, resulting in some ringing at the switch node. the ring frequency is set by switch capacitance, diode capacitance and induc- tance. this ringing has little energy, and its sinusoidal shape suggests it is free from harmonics. minimizing the copper area at the switch node will prevent this from causing interference problems. layout hints the lt1308a/LT1308B switch current at high speed, mandating careful attention to layout for proper perfor- mance. you will not get advertised performance with careless layout . figure 6 shows recommended component placement for an so-8 package boost (step-up) converter. follow this closely in your pc layout. note the direct path of the switching loops. input capacitor c1 must be placed close (< 5mm) to the ic package. as little as 10mm of wire or pc trace from c in to v in will cause problems such as inability to regulate or oscillation. the negative terminal of output capacitor c2 should tie close to the ground pin(s) of the lt1308a/LT1308B. doing this reduces di/dt in the ground copper which keeps high frequency spikes to a minimum. the dc/dc converter ground should tie to the pc board ground plane at one place only, to avoid introducing di/dt in the ground plane. 1 2 8 7 3 4 6 5 l1 c2 d1 lbo lbi lt1308a LT1308B v out v in gnd shutdown r1 r2 multiple vias ground plane 1308 f04 + c1 + figure 6. recommended component placement for so-8 package boost converter. note direct high current paths using wide pc traces. minimize trace area at pin 1 (v c ) and pin 2 (fb). use multiple vias to tie pin 4 copper to ground plane. use vias at one location only to avoid introducing switching currents into the ground plane figure 7 shows recommended component placement for a boost converter using the tssop package. placement is similar to the so-8 package layout. figure 5. converter waveforms in discontinuous mode v out 20mv/div v sw 10v/div i sw 500ma/div 500ns/div figure 4. 5v to 12v boost converter waveforms in continuous mode. 10 m f ceramic capacitor used at output v out 100mv/div v sw 10v/div i sw 1a/div 500ns/div
10 lt1308a/LT1308B figure 9. recommended component placement for sepic figure 8. sepic (single-ended primary inductance converter) converts 3v to 10v input to a 5v/500ma regulated output applicatio n s i n for m atio n wu u u 1 2 14 13 3 4 12 11 10 5 6 7 9 8 l1 c2 d1 lbo lbi lt1308a LT1308B v out v in gnd shutdown r1 r2 multiple vias ground plane 1308 f07 + c1 + a sepic (single-ended primary inductance converter) schematic is shown in figure 8. this converter topology produces a regulated output over an input voltage range that spans (i.e., can be higher or lower than) the output. recommended component placement for an so-8 pack- age sepic is shown in figure 9. 1 2 8 7 3 4 6 5 c3 l1a l1b d1 lbo lbi lt1308a LT1308B v out v in gnd shutdown r1 r2 ground plane 1308 f09 multiple vias + c2 c1 + v in sw fb LT1308B l1a ctx10-2 l1b d1 47k r2 100k r1 309k 680pf 1308a/b f08 c1 47 f c3 220 f 6.3v c2 4.7 f ceramic v out 5v 500ma v in 3v to 10v v c gnd shdn shutdown c1: avx tajc476m016 c2: taiyo yuden emk325bj475(x5r) c3: avx tpsd227m006 + + d1: ir 10bq015 l1: coiltronics ctx10-2 figure 7. recommended component placement for tssop boost converter. placement is similar to figure 4.
11 lt1308a/LT1308B applicatio n s i n for m atio n wu u u shdn pin the lt1308a/LT1308B shdn pin is improved over the lt1308. the pin does not require tying to v in to enable the device, but needs only a logic level signal. the voltage on the shdn pin can vary from 1v to 10v independent of v in . further, floating this pin has the same effect as grounding, which is to shut the device down, reducing current drain to 1 m a or less. low-battery detector the low-battery detector on the lt1308a/LT1308B fea- tures improved accuracy and drive capability compared to the lt1308. the 200mv reference has an accuracy of 2% and the open-collector output can sink 50 m a. the lt1308a/ LT1308B low-battery detector is a simple pnp input gain stage with an open-collector npn output. the negative input of the gain stage is tied internally to a 200mv reference. the positive input is the lbi pin. arrangement as a low-battery detector is straightforward. figure 10 details hookup. r1 and r2 need only be low enough in value so that the bias current of the lbi pin doesnt cause large errors. for r2, 100k is adequate. the 200mv refer- ence can also be accessed as shown in figure 11. lbo lbi to processor r1 100k r2 100k v in v bat lt1308a LT1308B 1308 f10 5v gnd 200mv internal reference + r1 = v lb ?200mv 2 m a v in v bat lt1308a LT1308B lbi lbo 200k 10 m f gnd 10k 1308 f11 2n3906 v ref 200mv + 195 200 205 v lbi (mv) 1308 f12 v lbo 1v/div figure 12. low-battery detector input/output characteristic figure 10. setting low-battery detector trip point figure 11. accessing 200mv reference a cross plot of the low-battery detector is shown in figure 12. the lbi pin is swept with an input which varies from 195mv to 205mv, and lbo (with a 100k pull-up resistor) is displayed. start-up the lt1308a/LT1308B can start up into heavy loads, unlike many cmos dc/dc converters that derive operat- ing voltage from the output (a technique known as bootstrapping). figure 13 details start-up waveforms of figure 1s circuit with a 20 w load and v in of 1.5v. inductor current rises to 3.5a as the output capacitor is charged. after the output reaches 5v, inductor current is about 1a. in figure 14, the load is 5 w and input voltage is 3v. output voltage reaches 5v in 500 m s after the device is enabled. figure 15 shows start-up behavior of figure 5s sepic circuit, driven from a 9v input with a 10 w load. the output reaches 5v in about 1ms after the device is enabled. v out 2v/div i l1 1a/div v shdn 5v/div 1ms/div 1308 f13 figure 13. 5v boost converter of figure 1. start-up from 1.5v input into 20 w load
12 lt1308a/LT1308B applicatio n s i n for m atio n wu u u when operating from a battery composed of alkaline cells. the inrush current may cause sufficiency internal voltage drop to trigger a low-battery indicator. a pro- grammable soft-start can be implemented with 4 discrete components. a 5v to 12v boost converter using the LT1308B is detailed in figure 16. c4 differentiates v out , causing a current to flow into r3 as v out increases. when this current exceeds 0.7v/33k, or 21 m a, current flows into the base of q1. q1s collector then pulls current out the v c pin, creating a feedback loop where the slope of v out is limited as follows: d d v t v kc out = 07 33 4 . with c4 = 33nf, v out /t is limited to 640mv/ms. start-up waveforms for figure 16s circuit are pictured in figure 17. without the soft-start circuit implemented, the inrush current reaches 3a. the circuit reaches final output voltage in approximately 250 m s. adding the soft-start components reduces inductor current to less than 1a, as detailed in figure 18, while the time required to reach final output voltage increases to about 15ms. c4 can be adjusted to achieve any output slew rate desired. v in sw LT1308B gnd v c fb shdn + 330pf 100k shutdown soft-start components c1 47 f c4 33nf q1 r4 33k r3 33k v in 5v d1 l1 4.7 h v out 12v 500ma c2 10 f c c 100pf 11.3k r c 47k 10k c1: avx taj476m010 c2: taiyo yuden tmk432bj106mm d1: ir 10bq015 l1: murata lqh6c4r7 q1: 2n3904 1308 f16 figure 16. 5v to 12v boost converter with soft-start components q1, c4, r3 and r4. soft-start in some cases it may be undesirable for the lt1308a/ LT1308B to operate at current limit during start-up, e.g., v out 2v/div 500 m s/div 1308 f15 i sw 2a/div v shdn 5v/div figure 15. 5v sepic start-up from 9v input into 10 w load v out 1v/div i l1 2a/div v shdn 5v/div 500 m s/div 1308 f14 figure 14. 5v boost converter of figure 1. start-up from 3v input into 5 w load
13 lt1308a/LT1308B so that copper loss is minimized. acceptable inductance values range between 2 m h and 20 m h, with 4.7 m h best for most applications. lower value inductors are physically smaller than higher value inductors for the same current capability. table 1 lists some inductors we have found to perform well in lt1308a/LT1308B application circuits. this is not an exclusive list. table 1 vendor part no. value phone no. murata lqh6c4r7 4.7 m h 770-436-1300 sumida cdrh734r7 4.7 m h 847-956-0666 coiltronics ctx5-1 5 m h 561-241-7876 coilcraft lpo2506ib-472 4.7 m h 847-639-6400 capacitors equivalent series resistance (esr) is the main issue regarding selection of capacitors, especially the output capacitors. the output capacitors specified for use with the lt1308a/ LT1308B circuits have low esr and are specifically designed for power supply applications. output voltage ripple of a boost converter is equal to esr multiplied by switch current. the performance of the avx tpsd227m006 220 m f tantalum can be evaluated by referring to figure 3. when the load is 800ma, the peak switch current is approximately 2a. output voltage ripple is about 60mv p- p , so the esr of the output capacitor is 60mv/2a or 0.03 w . ripple can be further reduced by paralleling ceramic units. table 2 lists some capacitors we have found to perform well in the lt1308a/LT1308B application circuits. this is not an exclusive list. table 2 vendor series part no. value phone no. avx tps tpsd227m006 220 m f, 6v 803-448-9411 avx tps tpsd107m010 100 m f, 10v 803-448-9411 taiyo yuden x5r lmk432bj226 22 m f, 10v 408-573-4150 taiyo yuden x5r tmk432bj106 10 m f, 25v 408-573-4150 applicatio n s i n for m atio n wu u u component selection diodes we have found on semiconductor mbrs130 and interna- tional rectifier 10bq015 to perform well. for applications where v out exceeds 30v, use 40v diodes such as mbrs140 or 10bq040. height limited applications may benefit from the use of the mbrm120. this component is only 1mm tall and offers performance similar to the mbrs130. inductors suitable inductors for use with the lt1308a/LT1308B must fulfill two requirements. first, the inductor must be able to handle current of 2a steady-state, as well as support transient and start-up current over 3a without inductance decreasing by more than 50% to 60%. second, the dcr of the inductor should have low dcr, under 0.05 w figure 17. start-up waveforms of figure 16s circuit without soft-start components figure 18. start-up waveforms of figure 16s circuit with soft-start components added v out 5v/div 50 m s/div 1308 f17 i l1 1a/div v shdn 10v/div v out 5ms/div 1308 f18 i l1 1a/div v shdn 10v/div 12v 5v 12v 5v
14 lt1308a/LT1308B v in sw LT1308B gnd v c fb shdn + c pl 330pf r1 100k c1 47 f v in 5v d1 l1 4.7 h v out 12v 500ma c2 100pf r2 11.3k 47k r3 10k c1: avx tajc476m010 c2: avx tpsd476m016 (47 f) or taiyo yuden tmk432bj106mm (10 f) d1: ir 10bq015 l1: murata lqh6c4r7 1308 f19 figure 19. 5v to 12v boost converter applicatio n s i n for m atio n wu u u ceramic capacitors multilayer ceramic capacitors have become popular, due to their small size, low cost, and near-zero esr. ceramic capacitors can be used successfully in lt1308a/LT1308B designs provided loop stability is considered. a tantalum capacitor has some esr and this causes an "esr zero" in the regulator loop. this zero is beneficial to loop stability. ceramics do not have appreciable esr, so the zero is lost when they are used. however, the lt1308a/LT1308B have external compensation pin (v c ) so component val- ues can be adjusted to achieve stability. a phase lead capacitor can also be used to tune up load step response to optimum levels, as detailed in the following paragraphs. figure 19 details a 5v to 12v boost converter using either a tantalum or ceramic capacitor for c2. the input capaci- tor has little effect on loop stability, as long as minimum capacitance requirements are met. the phase lead capaci- tor c pl parallels feedback resistor r1. figure 20 shows load step response of a 50ma to 500ma load step using a 47 m f tantalum capacitor at the output. without the phase lead capacitor, there is some ringing, suggesting the phase margin is low. c pl is then added, and response to the same load step is pictured in figure 21. some phase margin is restored, improving the response. next, c2 is replaced by a 10 m f, x5r dielectric, ceramic capacitor. without c pl , load step response is pictured in figure 22. although the output settles faster than the tantalum case, there is appreciable ringing, again suggesting phase mar- gin is low. figure 23 depicts load step response using the 10 m f ceramic output capacitor and c pl . response is clean and no ringing is evident. ceramic capacitors have the added benefit of lowering ripple at the switching frequency due to their very low esr. by applying c pl in tandem with the series rc at the v c pin, loop response can be tailored to optimize response using ceramic output capacitors. figure 21. load step response with 47 m f tantalum output capacitor and phase lead capacitor c pl v out 500mv/div 200 m s/div 1308 f20 figure 20. load step response of LT1308B 5v to 12v boost converter with 47 m f tantalum output capacitor v out 500mv/div 200 m s/div 1308 f21 v out 1v/div i l1 1a/div 200 m s/div 1308 f22 figure 22. load step response with 10 m f x5r ceramic output capacitor load current 50ma 500ma i l1 1a/div load current 50ma 500ma i l1 1a/div load current 50ma 500ma
15 lt1308a/LT1308B applicatio n s i n for m atio n wu u u v out v in = 4.2v v out v in = 3.6v v out v in = 3v i load 1a 10ma v out traces = 200 m s/div 200mv/div 1308 f25 figure 26. LT1308B li-ion to 5v boost converter transient response to 1a load step v out v in = 4.2v v out v in = 3.6v v out v in = 3v figure 25. lt1308a li-ion to 5v boost converter transient response to 1a load step v out traces = 100 m s/div 200mv/div 1308 f26 i load 1a 10ma figure 24. li-ion to 5v boost converter delivers 1a gsm and cdma phones the lt1308a/LT1308B are suitable for converting a single li-ion cell to 5v for powering rf power stages in gsm or cdma phones. improvements in the lt1308a/LT1308B error amplifiers allow external compensation values to be reduced, resulting in faster transient response compared to the lt1308. the circuit of figure 24 (same as figure 1, printed again for convenience) provides a 5v, 1a output from a li-ion cell. figure 25 details transient response at the lt1308a operating at a v in of 4.2v, 3.6v and 3v. ripple voltage in burst mode operation can be seen at 10ma load. figure 26 shows transient response of the LT1308B under the same conditions. note the lack of burst mode ripple at 10ma load. v in sw fb LT1308B l1 4.7 h d1 47k r2 100k r1 309k 5v 1a 100pf 1308a/b f24 c1 47 f c2 220 f li-ion cell v c gnd shdn shutdown c1: avx tajc476m010 c2: avx tpsd227m006 + + d1: ir 10bq015 l1: murata lqh6n4r7 v out 500mv/div 200 m s/div 1308 f23 i l1 1a/div load current 50ma 500ma figure 23. load step response with 10 m f x5r ceramic output capacitor and c pl
16 lt1308a/LT1308B typical applicatio s u triple output tftlcd bias supply d1 d4 0.22 f l1 4.7 h 65 2 4 3 1 v in sw LT1308B gnd v c fb shdn 0.22 f 220k 10.7k 1308 ta02 76.8k c1 4.7 f v in 5v c2, c3 10 f 2 c6 1 f c5 1 f c4 1 f 0.22 f av dd 10v 500ma v on 27v 15ma v off ?v 10ma 100pf d3 d2 c1:taiyo-yuden jmk212bj475mg c2, c3:taiyo-yuden lmk325bj106mn c4, c5, c6:taiyo-yuden emk212bj105mg d1: mbrm120 d2,d3,d4: bat54s l1: toko 817fy-4r7m av dd 500mv/div v on 500mv/div v off 500mv/div 100 s/div i load 800ma 200ma tftlcd bias supply transient response
17 lt1308a/LT1308B d4 v in sw lt1308a gnd v c fb shdn 47k 1308 ta04 c1 47 f 10nf 100pf d2 d1 d3 34.8k 10m 10nf 250v 10nf 250v 10nf 250v + v out 350v 1.2ma v in 2.7v to 6v t1 1:12 3 1 4 6 shutdown d1, d2, d3: bav21 200ma, 250v d4: mbr0540 t1: midcom 31105r l p = 1.5 h v in sw fb LT1308B l1a ctx10-2 l1b d1 47k r2 100k r1 309k 680pf 1308a/b ta05 c1 47 f c3 220 f 6.3v c2 4.7 f ceramic v out 5v 500ma v in 3v to 10v v c gnd shdn shutdown c1: avx tajc476m016 c2: taiyo yuden emk325bj475(x5r) c3: avx tpsd227m006 + + d1: ir 10bq015 l1: coiltronics ctx10-2 sepic converts 3v to 10v input to a 5v/500ma regulated output 40nf el panel driver high voltage supply 350v at 1.2ma typical applicatio s u 47pf 10k 47k 17k c1 47 f v bat 3v to 6v 100pf d3 d2 d1 shutdown 2m 4.3m 1 f q1 100k 150k 324k 3.3k 22nf 49.9k q2 400v 1308 ta03 c2 1 f 200v el panel 40nf + 3 1 4 6 v in sw lt1308a gnd v c fb lbo lbi shdn 3.3v regulated t1 1:12 c1: avx tajc476m010 c2: vitramon vj225y105kxcat d1: bat54 d2, d3: bav21 q1: mmbt3906 q2: zetex fcx458 t1: midcom 31105
18 lt1308a/LT1308B package descriptio n u dimensions in inches (millimeters) unless otherwise noted. s8 package 8-lead plastic small outline (narrow 0.150) (ltc dwg # 05-08-1610) 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) so8 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) 8 7 6 5 0.189 ?0.197* (4.801 ?5.004) 0.228 ?0.244 (5.791 ?6.197) 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 * **
19 lt1308a/LT1308B package descriptio n u dimensions in inches (millimeters) unless otherwise noted. f14 tssop 1299 0.09 ?0.18 (0.0035 ?0.0071) 0 ?8 0.50 ?0.70 (0.020 ?0.028) 4.30 ?4.48** (0.169 ?0.176) 6.25 ?6.50 (0.246 ?0.256) 134 5 6 7 8 4.90 ?5.10* (0.193 ?0.201) 14 13 12 11 10 9 1.10 (0.0433) max 0.05 ?0.15 (0.002 ?0.006) 0.65 (0.0256) bsc 0.18 ?0.30 (0.0071 ?0.0118) 2 note: dimensions are in millimeters dimensions do not include mold flash. mold flash shall not exceed 0.152mm (0.006") per side dimensions do not include interlead flash. interlead flash shall not exceed 0.254mm (0.010") per side * ** f package 14-lead plastic tssop (4.4mm) (ltc dwg # 05-08-1650) 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.
20 lt1308a/LT1308B part number description comments lt1302 high output current micropower dc/dc converter 5v/600ma from 2v, 2a internal switch, 200 m a i q lt1304 2-cell micropower dc/dc converter 5v/200ma, low-battery detector active in shutdown lt1307/lt1307b single cell, micropower, 600khz pwm dc/dc converters 3.3v at 75ma from one cell, msop package lt1316 burst mode operation dc/dc with programmable current limit 1.5v minimum, precise control of peak current limit lt1317/lt1317b micropower, 600khz pwm dc/dc converters 100 m a i q , operate with v in as low as 1.5v ltc ? 1474 micropower step-down dc/dc converter 94% efficiency, 10 m a i q , 9v to 5v at 250ma ltc1516 2-cell to 5v regulated charge pump 12 m a i q , no inudctors, 5v at 50ma from 3v input ltc1522 micropower, 5v charge pump dc/dc converter regulated 5v 4% output, 20ma from 3v input lt1610 single-cell micropower dc/dc converter 3v at 30ma from 1v, 1.7mhz fixed frequency lt1611 inverting 1.4mhz switching regulator in 5-lead sot-23 C 5v at 150ma from 5v input, tiny sot-23 package lt1613 1.4mhz switching regulator in 5-lead sot-23 5v at 200ma from 4.4v input, tiny sot-23 package lt1615 micropower step-up dc/dc in 5-lead sot-23 20 m a i q , 36v, 350ma switch lt1617 micropower inverting dc/dc converter in sot-23 v in = 1v to 15v; v out to C34v ltc1682 doubler charge pump with low noise ldo adjustable or fixed 3.3v, 5v outputs, 60 m v rms output noise lt1949 600khz, 1a switch pwm dc/dc converter 1.1a, 0.5 w , 30v internal switch, v in as low as 1.5v lt1949-1 1.1mhz, 1a switch dc/dc converter 1.1mhz version of lt1949 ? linear technology corporation 1999 1308abf lt/tp 0800 4k ? printed in usa related parts linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear-tech.com li-ion to 12v/300ma step-up dc/dc converter typical applicatio u v in sw fb LT1308B l1 4.7 h d1 47k r2 100k r1 887k 12v 300ma 330pf 1308a/b ta01 c1 47 f c2 100 f li-ion cell v c gnd shdn shutdown c1: avx tajc476m010 c2: avx tpsd107m016 d1: ir 10bq015 + + l1: murata lqh6c4r7 2.7v to 4.2v

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