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1 ltc1734l 1734lf lithium-ion linear battery charger in thinsot the ltc 1734l is a low cost, single cell, constant-current/ constant-voltage li-ion battery charger controller. whencombined with a few external components, the sot-23 package forms a very small, low cost charger for single cell lithium-ion batteries. the ltc1734l is a lower charge current version of the ltc1734. the ltc1734l provides a fixed float voltage of 4.2v with 1% accuracy (for 4.1v and 4.15v float voltages, contact ltc marketing). constant current is programmed using a single external resistor between the prog pin and ground. manual shutdown is accomplished by floating the pro- gram resistor while removing input power automatically puts the ltc1734l into a sleep mode. both the shutdown and sleep modes drain near zero current from the battery. charge current can be monitored via the voltage on the prog pin allowing a microcontroller or adc to read the current and determine when to terminate the charge cycle. the output driver is both current limited and thermally protected to prevent the ltc1734l from operating outside of safe limits. no external blocking diode is required. the ltc1734l can also function as a general purpose current source or as a current source for charging nickel- cadmium (nicd) and nickel-metal-hydride (nimh) batter- ies using external termination. low profile (1mm) thinsot tm package programmable charge current: 50ma to 180ma no blocking diode required no sense resistor required 1% accurate preset voltage: 4.2v charge current monitor outputfor charge termination automatic sleep mode with input supply removal manual shutdown negligible battery drain current in shutdown undervoltage lockout self protection for overcurrent/overtemperature cellular telephones handheld computers digital cameras charging docks and cradles low cost and small size chargers programmable current sources , ltc and lt are registered trademarks of linear technology corporation. 80ma li-ion battery charger prog pin indicates charge status features descriptio u applicatio s u typical applicatio u v in 5v i bat = 80ma 1734 ta01 singleli-ion battery 10 f + gndprog drive bat 24 65 ltc1734l v cc i sense 31 1 f r prog 4.7k umt4403 charging begins charging complete 1734 ta01b 5v4v 3v 2v 1v 0v 1.5v v bat constant current v bat (v) v prog (v) constant voltage v prog thinsot is a trademark of linear technology corporation. downloaded from: http:///
2 ltc1734l 1734lf order part number (note 1)input supply voltage (v cc ) .......................... 0.3v to 9v input voltage (bat, prog) ........ 0.3v to (v cc + 0.3v) output voltage (drive) .............. 0.3v to (v cc + 0.3v) output current (i sense ) ................................... 210ma short-circuit duration (drive) ...................... indefinite junction temperature .......................................... 125 c operating ambient temperature range (note 2) ...............................................40 c to 85 c operating junction temperature (note 2) ............ 100 c storage temperature range ................. 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c ltc1734les6-4.2 t jmax = 125 c, q ja = 230 c/w absolute m axi m u m ratings w ww u package/order i n for m atio n w u u s6 part marking lte6 the denotes the specifications which apply over the full operating temperature range, unless otherwise noted specifications are at t a = 25 c. v cc = 5v, gnd = 0v and v bat is equal to the float voltage unless otherwise noted. all current into a pin is positive and current out of a pin is negative. all voltages are referenced to gnd, unless otherwise specified. electrical characteristics i sense 1 gnd 2 v cc 3 6 drive5 bat 4 prog top view s6 package 6-lead plastic sot-23 consult ltc marketing for parts specified with wider operating temperature ranges. symbol parameter conditions min typ max units v cc supply v cc input supply voltage (note 5) 4.55 8 v i cc quiescent v cc pin supply current v bat = 5v, (forces i drive = i bat = 0), 670 1150 m a i prog = 200 m a,(7500 w from prog to gnd) i shdn v cc pin supply current in manual shutdown prog pin open 450 900 m a i bms battery drain current in manual shutdown prog pin open ? 0 1 m a (note 3) i bsl battery drain current in sleep mode (note 4) v cc = 0v ? 0 1 m a v uvloi undervoltage lockout exit threshold v cc increasing 4.45 4.56 4.68 v v uvlod undervoltage lockout entry threshold v cc decreasing 4.30 4.41 4.53 v v uvhys undervoltage lockout hysteresis v cc decreasing 150 mv charging performance v bat output float voltage in constant voltage mode i bat = 10ma, 4.55v v cc 8v 4.158 4.20 4.242 v i bat1 output full-scale current when programmed r prog = 7500 w , 4.55v v cc 8v, 39 50 60 ma for 50ma in constant current mode pass pnp beta > 50 i bat2 output full-scale current when programmed r prog = 2100 w , 4.55v v cc 8v, 160 180 200 ma for 180ma in constant current mode pass pnp beta > 50 v cm1 current monitor voltage on prog pin i bat = 10% of i bat1 , r prog = 7500 w , 0.045 0.15 0.28 v 4.55v v cc 8v, pass pnp beta > 50, 0 c t a 85 c v cm2 current monitor voltage on prog pin i bat = 10% of i bat2 , r prog = 2100 w , 0.10 0.15 0.20 v 4.55v v cc 8v, pass pnp beta > 50, 0 c t a 85 c i dsink drive output current v drive = 3.5v 20 ma downloaded from: http:///
3 ltc1734l 1734lf the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v cc = 5v, gnd = 0v and v bat is equal to the float voltage unless otherwise noted. all current into a pin is positive and current out of a pin is negative. all voltages are referenced to gnd, unless otherwise specified. electrical characteristics symbol parameter conditions min typ max units charger manual control v msdt manual shutdown threshold v prog increasing 2.05 2.15 2.25 v v mshys manual shutdown hysteresis v prog decreasing from v msdt 90 mv i progpu program pin pull-up current v prog = 2.5v 6 3 1.5 m a protection i dshrt drive output short-circuit current limit v drive = v cc 35 65 130 ma note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired.note 2: the ltc1734le is guaranteed to meet performance specifications from 0 c to 70 c ambient temperature range and 0 c to 100 c junction temperature range. specifications over the 40 c to 85 c operating ambient temperature range are assured by design, characterization andcorrelation with statistical process controls. note 3: assumes that the external pnp pass transistor has negligible b-c reverse-leakage current when the collector is biased at 4.2v (v bat ) and the base is biased at 5v (v cc ). note 4: assumes that the external pnp pass transistor has negligible b-e reverse-leakage current when the emitter is biased at 0v (v cc ) and the base is biased at 4.2v (v bat ). note 5: the 4.68v maximum undervoltage lockout (uvlo) exit threshold must first be exceeded before the minimum v cc specification applies. short duration drops below the minimum v cc specification of several microseconds or less are ignored by the uvlo. if manual shutdown isentered, then v cc must be higher than the 4.68v maximum uvlo threshold before manual shutdown can be exited. when operating near theminimum v cc , a suitable pnp transistor with a low saturation voltage must be used. typical perfor a ce characteristics uw float voltage vs temperatureand supply voltage temperature ( c) ?0 4.19 float voltage (v) 4.20 4.21 0 50 75 1734 g01 ?5 25 100 125 i bat = 10ma pnp = fcx589 v cc = 8v v cc = 4.55v i bat (ma) 0 4.199 float voltage (v) 4.200 4.201 50 100 125 1734 g02 25 75 150 175 v cc = 5v t a = 25 c pnp = fcx589r prog = 2100 temperature ( c) ?0 48 49 i bat1 (ma) 50 51 52 0 50 75 1734 g03 ?5 25 100 125 r prog = 7.5k pnp = fcx589 v cc = 4.55v and 8v float voltage vs i bat i bat1 vs temperature and supply voltage downloaded from: http:///
4 ltc1734l 1734lf typical perfor a ce characteristics uw i bat2 vs temperature and supply voltage i bat1 vs v bat temperature ( c) ?0 170 i bat2 (ma) 180 190 0 50 75 1734 g04 ?5 25 100 125 r prog = 2.1k pnp = fcx589 v cc = 4.55v and 8v v bat (v) 0 i bat1 (ma) 52 4 1734 g05 5048 1 2 3 5 v cc = 5v t a = 25 c r prog = 7.5k pnp = fcx589 bat pin must be disconnected and grounded to force cc mode in this region i bat2 vs v bat v bat (v) 0 i bat2 (ma) 190 4 1734 g06 180170 1 2 3 5 v cc = 5v t a = 25 c r prog = 2.1k pnp = fcx589 bat pin must be disconnected and grounded to force cc mode in this region program pin pull-up current vstemperature and supply voltage program pin pull-up currentvs v prog program pin voltagevs charge current (50ma) temperature ( c) ?0 i progpu ( a) 3.4 3.5 3.6 25 75 1734 g07 3.3 3.2 ?5 0 50 100 125 3.1 3.0 v prog = 2.5v v cc = 8v v cc = 4.55v v prog (v) 2 2.6 i progpu ( a) 2.8 3.0 3.2 3.4 3.6 3 456 1635 g08 78 v cc = 8v t a = 25 c i bat1 (ma) 0 v prog (v) 0.8 1.0 1.2 50 1734 f09 0.60.4 0 10 20 40 30 0.2 1.61.4 v cc = 5v t a = 25 c r prog = 7.5k pnp = fcx589 limits at 25mv due toprogramming pin pull-up current (i progpu ) program pin voltagevs charge current (180ma) program pin voltage for i bat1 /10 vs temperature and supply voltage i bat2 (ma) 0 v prog (v) 1.4 1734 g10 0.80.4 45 90 0.2 0 1.61.2 1.0 0.6 135 180 v cc = 5v t a = 25 c r prog = 2.1k pnp = fcx589 limits at 6mv due toprogramming pin pull-up current (i progpu ) temperature ( c) ?0 140 v prog (mv) 150 160 0 50 75 1734 g11 ?5 25 100 125 r prog = 7.5k pnp = fcx589 v cc = 8v v cc = 4.55v program pin voltage for i bat2 /10 vs temperature and supply voltage temperature ( c) ?0 140 v prog (mv) 150 160 0 50 75 1734 g12 ?5 25 100 125 r prog = 2.1k pnp = fcx589 v cc = 8v v cc = 4.55v downloaded from: http:///
5 ltc1734l 1734lf block diagra w + + + 2 5 uvlo voltage reference 2.5v shutdown shutdown shutdown shutdown gnd 1734 bd 4 prog r prog 2.5v 10 f a1 a2 a3 1.5v + c1 2.15v 3 a bat i bat singleli-ion cell 6 drive 1 i sense v cc 1 f v in i bat /250 temperature and current limiting 60 0.24 3 ref output driver i bat pi n fu n ctio n s uuu i sense (pin 1): sense node for charge current. current from v cc passes through the internal current sense resis- tor and reappears at i sense to supply current to the external pnp emitter. the pnp collector provides chargecurrent to the battery. gnd (pin 2): ground. provides a reference for the internal voltage regulator and a return for all internal circuits.when in the constant voltage mode, the ltc1734l will precisely regulate the voltage between the bat and gnd pins. the battery ground should connect close to the gnd pin to avoid voltage drop errors. v cc (pin 3): positive input supply voltage. supplies power to the internal control circuitry and external pnptransistor through the internal current sense resistor. this pin should be bypassed to ground with a capacitor in the range of 1 m f to 10 m f. prog (pin 4): charge current programming, charge current monitor and manual shutdown pin. provides avirtual reference voltage of 1.5v for an external resistor (r prog ) tied between this pin and ground that programs the battery charge current when the charger is in theconstant current mode. the typical charge current will be 250 times greater than the current through this resistor (i bat = 375/r prog ). this pin also allows for the charge current to be monitored. the voltage on this pin is propor-tional to the charge current where 1.5v corresponds to the full programmed currrent. floating this pin allows an internal current source to pull the pin voltage above the shutdown threshold voltage. because this pin is in a signal path, excessive capacitive loading can cause ac instabil- ity. see the applications information section for more details. bat (pin 5): battery voltage sense input. a precision internal resistor divider sets the final float voltage on thispin. this divider is disconnected in the manual shutdown or sleep mode. when charging, approximately 34 m a flows into the bat pin. to minimize float voltage errors,avoid excessive resistance between the battery and the bat pin. for dynamically stable operation, this pin usually requires a minimum bypass capacitance to ground of 5 m f to frequency compensate for the high frequency inductiveeffects of the battery and wiring. drive (pin 6): base drive output for the external pnp pass transistor. provides a controlled sink current thatdrives the base of the pnp. this pin has current limiting protection for the ltc1734l. downloaded from: http:///
6 ltc1734l 1734lf a1? input) the amplifier will divert current away from theoutput driver thus maintaining 4.2v on the battery. this is the constant voltage mode. when in the constant voltage mode, the 250:1 current ratio is still valid and the voltage on the prog pin will indicate the charge current as a proportion of the maximum cur- rent set by the current programming resistor. the battery charge current is 250 ?(v prog /r prog ) amps. this feature allows a microcontroller with an adc to easily monitorcharge current and if desired, manually shut down the charger at the appropriate time. when v cc is applied, the charger can be manually shut down by floating the otherwise grounded end of r prog . an internal 3 m a current source pulls the prog pin above the 2.15v threshold of voltage comparator c1 initiatingshutdown. for charging nimh or nicd batteries, the ltc1734l can function as a constant current source by grounding the bat pin. this will prevent amplifier a1 from trying to limit charge current and only a2 will control the current. fault conditions such as overheating of the die or exces- sive drive pin current are monitored and limited. when input power is removed or manual shutdown is entered, the charger will drain only tiny leakage currents from the battery, thus maximizing battery standby time. with v cc removed the external pnp? base is connected to the battery by the charger. in manual shutdown the baseis connected to v cc by the charger. operatio u the ltc1734l is a linear battery charger controller.operation can best be understood by referring to the block diagram. charging begins when v cc rises above the uvlo (undervoltage lockout) threshold v uvloi and an external program resistor is connected between theprog pin and ground. when charging, the collector of the external pnp provides the charge current. the pnp? emitter current flows through the i sense pin and through the internal 0.24 w current sense resistor. this current is close in magnitude, but slightly more than the collectorcurrent since it includes the base current. amplifier a3, along with the p-channel fet, will force the same voltage that appears across the 0.24 w resistor to appear across the internal 60 w resistor. the scale factor of 250:1 in resistor values will cause the fet? drain current to be 1/250 of the charge current and it is this current that flows through the prog pin. in the constant current mode, amplifier a2 is used to limit the charge current to the value that is programmed by r prog . the prog pin current, which is 1/250 of the chargecurrent, develops a voltage across the program resistor. when this voltage reaches 1.5v, amplifier a2 begins diverting current away from the output driver, thus limit- ing the charge current. this is the constant current mode. the constant charge current is 250 ?(1.5v/r prog ). as the battery accepts charge, its voltage rises. when itreaches the preset float voltage of 4.2v, a precisely divided down version of this voltage (2.5v) is compared to the 2.5v internal reference voltage by amplifier a1. if the battery voltage attempts to exceed 4.2v (2.5v at amplifier applicatio s i for atio wu uu charger operation charging begins when an input voltage is present thatexceeds the undervoltage lockout threshold (v uvloi ), a li-ion battery is connected to the charger output and aprogram resistor is connected from the prog pin to ground. during the first portion of the charge cycle, when the battery voltage is below the preset float voltage, the charger is in the constant current mode. as the battery voltage rises and reaches the preset float voltage, the charge current begins to decrease and the constantvoltage portion of the charge cycle begins. the charge current will continue to decrease exponentially as the battery approaches a fully charged condition. should the battery be removed during charging, a fastbuilt-in protection circuit will prevent the bat pin from ris- ing above 5v, allowing the precision constant voltage circuit time to respond. downloaded from: http:///
7 ltc1734l 1734lf applicatio n s i n for m atio n wu u u manual shutdownfloating the program resistor allows an internal 3 m a current source (i progpu ) to pull the prog pin above the 2.15v shutdown threshold (v msdt ), thus shutting down the charger. in this mode, the ltc1734l continues todraw quiescent current from the supply (i shdn ), but only a negligible leakage current is delivered to the battery(i bms ). shutdown can also be accomplished by pulling the other-wise grounded end of the program resistor to a voltage greater than 2.25v (v msdt max). charging will cease above 1.5v, but the internal battery voltage resistor divider willdraw about 34 m a from the battery until shutdown is entered. figure 1 illustrates a microcontroller configura-tion that can either float the resistor or force it to a voltage. the voltage should be no more than 8v when high and have an impedance to ground of less than 10% of the program resistor value when low to prevent excessive charge current errors. to reduce errors the program resistor value may be adjusted to account for the imped- ance to ground. the programming resistor will prevent potentially damaging currents if the prog pin is forced above v cc . under this condition v cc may float, be loaded down by other circuitry or be shorted to ground. if v cc is not shorted to ground, the current through the resistor willpull v cc up slightly. another method is to directly switch the prog pin to avoltage source when shutdown is desired (caution: pull- ing the prog below 1.5v with v cc applied will cause excessive and uncontrolled charge currents). the volt-age source must be capable of sourcing the resulting current through the program resistor. this has the ad- vantage of not adding any error to the program resistor during normal operation. the voltage on the prog pin must be greater than 2.25v (v msdt(max) ) to ensure entering shutdown, but no more than 0.3v above v cc to prevent damaging the ltc1734l from excessive progpin current. an exception is if v cc is allowed to float with no other circuitry loading v cc down. then, because the current will be low, it is allowable to have the prog pinshutdown voltage applied. a three-state logic driver with sufficient pull-up current can be used to perform this function by enabling the high impedance state to charge or enabling the pull-up device to enter shutdown. an npn transistor or a diode can also be utilized toimplement shutdown from a voltage source. these have the advantage of blocking current when the voltage source goes low, thus automatically disconnecting the voltage source for normal charging operation. using an npn allows the use of a weak voltage source due to the current gain of the transistor. for an npn, connect the collector to v cc, the base to the voltage source and the emitter to the prog pin. for a diode, connect the anode to the voltagesource and cathode to the prog pin. an input high level ranging from 3.3v to v cc should be adequate to enter shutdown while a low level of 0.5v or less should allow fornormal charging operation. use of inexpensive small signal devices such as the 2n3904 or 1n914 is recom- mended to prevent excessive capacitive loading on the prog pin (see stability section). sleep mode when the input supply is disconnected, the ic enters the sleep mode. in this mode, the battery drain current (i bsl ) is a negligible leakage current, allowing the battery to re-main connected to the charger for an extended period of time without discharging the battery. the leakage current is due to the reverse-biased b-e junction of the external pnp transistor. undervoltage lockout undervoltage lockout (uvlo) keeps the charger off until the input voltage exceeds a predetermined threshold level (v uvloi ) that is typically 4.56v. approximately 150mv of hysteresis is built in to prevent oscillation around thethreshold level. in undervoltage lockout, battery drain current is very low (< 1 m a). figure 1. interfacing with a microcontroller ltc1734l prog c r prog adc input 1734 f01 open drain or totem pole output downloaded from: http:///
8 ltc1734l 1734lf applicatio n s i n for m atio n wu u u programming constant current when in the constant current mode, the full-scale chargecurrent is programmed using a single external resistor between the prog pin and ground. this charge current will be 250 times the current through the program resis- tor. the program resistor value is selected by dividing the voltage forced across the resistor (1.5v) by the desired resistor current: r prog = 375/i bat the ltc1734l is designed for an absolute maximumcurrent of 210ma. this translates to a maximum prog pin current of 840 m a and a minimum program resistor of 1.8k. because the prog pin is in a closed-loop signal path, thepole frequency must be kept high enough to maintain adequate ac stability by avoiding excessive capacitance on the pin. see the stability section for more details. the minimum full-scale current that can be reliably pro-grammed is approximately 10ma, which requires a pro- gram resistor of 37.4k. limiting capacitive loading on the program pin becomes more important when high value program resistors are used. in addition, the current monitoring accuracy can degrade considerably at verylow current levels. if current monitoring is desired, a minimum full-scale current of 50ma is recommended. different charge currents can be programmed by variousmeans such as by switching in different program resistors as shown in figures 2 and 3. a voltage dac connected through a resistor to the prog pin or a current dac connected in parallel with a resistor to the prog pin can also be used to program current (the resistor is required with the i dac to maintain ac stability as discussed in the stability section). another means is to use a pwm outputfrom a microcontroller to duty cycle the charger into and out of shutdown to create an average current (see manual shutdown section for interfacing examples). because chargers are generally slow to respond, it can take up to approximately 300 m s for the charger to fully settle after a shutdown is deasserted. this delay must be accounted forunless the minimum pwm low duration is about 3ms or more. shutdown occurs within a few microseconds of a shutdown command. the use of pwm can extend the average current to less than the normal 50ma minimum constant current. v in 5v charge currentmonitor (unfiltered) charge currentmonitor (filtered) 7.5k q22n7002 control 2 fzt549 i bat 1734 f02 singleli-ion battery 10 f gndprog drive bat 24 65 ltc1734l v cc i sense 31 1 f 0.1 f to 0.5 f 1k optional filter 3k pin 4 q12n7002 control 1 charge current 0 50ma 125ma175ma control 1 lowlow highhigh control 2 low high low high v in 5v 7.5k q22n7002 control 2 fzt549* i load *observe maximum temperature 1734 f03 gndprog drive bat 24 65 ltc1734l v cc i sense 31 1 f 3k q12n7002 control 1 current 0 50ma 125ma175ma control 1 lowlow highhigh control 2 low high low high load figure 2. logic control programming of output current to 0ma, 50ma, 125ma or 175ma figure 3. programmable current source with output current of 0ma, 50ma, 125ma or 175ma downloaded from: http:///
9 ltc1734l 1734lf monitoring charge currentthe voltage on the prog pin indicates the charge current as a proportion of the maximum current set by the program resistor. the charge current is equal to 250 (v prog /r prog ) amps. this feature allows a microcontrol- ler with an adc to easily monitor charge current and ifdesired, manually shut down the charger at the appropri- ate time. the minimum prog pin current is about 3 m a (i progpu ). errors in the charge current monitor voltage on the progpin and in the full-scale charge current are inversely proportional to battery current and can be statistically approximated as follows: one sigma error(%) @ 1 + 0.08/i bat (a) dynamic loads on the battery will cause transients toappear on the prog pin. should they cause excessive errors in charge current monitoring, a simple rc filter as shown in figure 2 can be used to filter the transients. the filter will also quiet the prog pin to help prevent momen- tary entry into the manual shutdown mode. because the prog pin is in a closed-loop signal path the pole frequency must be kept high enough to maintain adequate ac stability. this means that the maximum resistance and capacitance presented to the prog pin must be limited. see the stability section for more details. constant current source the ltc1734l can be used as a constant current source by disabling the voltage control loop as shown in figure 3. this is done by pulling the bat pin below the preset float voltage of 4.2v by grounding the bat pin. the program resistor will determine the output current. the output current range can be between approximately 10ma and 180ma, depending on the maximum power rating of the external pnp pass transistor. external pnp transistor the external pnp pass transistor must have adequatebeta, low saturation voltage and sufficient power dissipa- tion capability (including any heat sinking, if required). to provide 180ma of charge current with the minimumavailable base drive of approximately 20ma requires a pnp beta greater than 9. with low supply voltages, the pnp saturation voltage(v cesat ) becomes important. the v cesat must be less than the minimum supply voltage minus the maximumvoltage drop across the internal sense resistor and bond wires (0.3 w ) and battery float voltage. if the pnp transis- tor can not achieve the low saturation voltage required,base current will dramatically increase. this is to be avoided for a number of reasons: output drive may reach current limit resulting in the charger? characteristics to go out of specifications, excessive power dissipation may force the ic into thermal shutdown, or the battery could become discharged because some of the current from the drive pin could be pulled from the battery through the forward biased collector base junction. for example, to program a charge current of 100ma witha minimum supply voltage of 4.75v, the minimum operat- ing v ce is: v ce(min) (v) = 4.75 ?(0.1)(0.3) ?4.2 = 0.52v the actual battery charge current (i bat ) is slightly less than the expected charge current because the chargersenses the emitter current and the battery charge current will be reduced by the base current. in terms of b (i c /i b ), i bat can be calculated as follows: i bat (a) = 250 ?i prog [ b /( b + 1)] if b = 50, then i bat is 2% low. if desired, the 2% loss can be compensated for by increasing i prog by 2%. another important factor to consider when choosing thepnp pass transistor is the power handling capability. the transistor? data sheet will usually give the maximum rated power dissipation at a given ambient temperature with a power derating for elevated temperature operation. the maximum power dissipation of the pnp when charging is: p d(max) (w) = i bat (v dd(max) ?v bat(min) ) v dd(max) is the maximum supply voltage and v bat(min) is the minimum battery voltage when discharged. applicatio n s i n for m atio n wu u u downloaded from: http:///
10 ltc1734l 1734lf once the maximum power dissipation and v ce(min) are known, table 1 can be used as a guide in selecting asuitable pnp transistor. in the table, very low v cesat is less than 0.25v, low v cesat is 0.25v to 0.5v and the others are 0.5v to 0.8v all depending on the current required. seethe manufacturer? data sheet for details. all of the pnp transistors are rated to carry at least 1a continuously as long as the power dissipation is within limits. the stability section addresses caution in the use of high beta pnps. should overheating of the pnp transistor be a concern, protection can be achieved with a positive temperature coefficient (ptc) thermistor, wired in series with the current programming resistor and thermally coupled to the transistor. the prf chip series from murata has a steep resistance increase at temperature thresholds from 85 c to 145 c making it behave somewhat like a thermo- stat switch. for example, the model prf18ba471qb1rbthermistor is 470 w at 25 c, but abruptly increase its resistance to 4.7k at 125 c. below 125 c, the device exhibits a small negative tc. the 470 w thermistor can be added in series with a 1.6k resistor to form the currentprogramming resistor for a 180ma charger. should the thermistor reach 125 c, the charge current will drop to 60ma and inhibit any further increase in temperature.stability the ltc1734l contains two control loops: constant volt- age and constant current. to maintain good ac stability in the constant voltage mode, a capacitor of at least 4.7 m f is usually required from bat to ground. the battery andinterconnecting wires appear inductive at high frequen- cies, and since these are in the feedback loop, this capaci- tance may be necessary to compensate for the inductance. this capacitor need not exceed 100 m f and its esr can range from near zero to several ohms depending on theinductance to be compensated. in general, compensation is optimal with a capacitance of 4.7 m f to 22 m f and an esr of 0.5 w to 1.5 w . using high beta pnp transistors (>300) and very low esr output capacitors (especially ceramic) reduces the phase margin, possibly resulting in oscillation. also, using high value capacitors with very low esrs will reduce the phase margin. adding a resistor of 0.5 w to 1.5 w in series with the capacitor will restore the phase margin.in the constant current mode, the prog pin is in the feedback loop, not the battery. because of this, capaci- tance on this pin must be limited. locating the program resistor near the prog pin and isolating the charge current monitoring circuitry (if used) from the prog pin with a 1k to 10k resistor may be necessary if the capaci- tance is greater than that given by the following equation: c k r max pf prog () = 400 applicatio n s i n for m atio n wu u u table 1. pnp pass transistor selection guide maximum p d (w) mounted on board at t a = 25 c package style zetex part number rohm part number comments 0.2 sc-70 umt4403 smallest size 0.2 sc-70 umt2907a smallest size 0.5 sot-23 fmmt549 low v cesat 0.625 sot-23 fmmt720 very low v cesat, high beta 1 sot-89 fcx589 or bcx69 1.1 sot-23-6 zxt10p12de6 very low v cesat, high beta, small 1 to 2 sot-89 fcx717 very low v cesat, high beta 2 sot-223 fzt589 low v cesat 2 sot-223 bcp69 or fzt549 0.75 ftr 2sb822 low v cesat 1 atv 2sb1443 low v cesat 2 sot-89 2sa1797 low v cesat downloaded from: http:///
11 ltc1734l 1734lf higher charge currents require lower program resistorvalues which can tolerate more capacitive loading on the prog pin. maximum capacitance can be as high as 50pf for a charge current of 50ma (r prog = 7.5k). figure 4 is a simple test circuit for checking stability in boththe constant current and constant voltage modes. with input power applied and a near fully charged battery connected to the charger, driving the prog pin with a pulse generator will cycle the charger in and out of the manual shutdown mode. referring to figure 5, after a short delay, the charger will enter the constant current mode first, then if the battery voltage is near the pro- grammed voltage of 4.2v, the constant voltage mode will begin. the resulting waveform on the prog pin is an indication of stability. the double exposure photo in figure 5 shows the effects of capacitance on the program pin. the middle waveform is typical while the lower waveform indicates excessive program pin capacitance resulting in constant current mode instability. although not common, ringing on the constant voltage portion of the waveform is an indication applicatio n s i n for m atio n wu u u 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. of instability due to any combination of extremely low esrvalues, high capacitance values of the output capacitor or very high pnp transistor beta. to minimize the effect of the scope probe capacitance, a 10k resistor is used to isolate the probe from the program pin. also, an adjustable load resistor or current sink can be used to quickly alter the charge current when a fully charged battery is used. reverse input voltage protection in some applications, protection from reverse voltage on v cc is desired. if the supply voltage is high enough, a series blocking diode can be used. in other cases, wherethe voltage drop must be kept low, a p-channel fet as shown in figure 6 can be used. figure 5. stability waveforms ltc1734l prog li-ion* 20 to 200 *fully charged cell 10k r prog 3k to scope 1734 f04 bat 2.5v f = 1khz 0v + figure 4. setup for ac stability testing 5v0v prog pin (20pf on pin) prog pin (200pf on pin) pulse generator 2v1v 0v 2v shut down delay constant current horizontal scale: 100 s/div 1v0v constant voltage v cc v in * 1734 f06 ltc1734l *drain-bulk diode of fet figure 6. low loss reverse voltage protection v cc bypass capacitor many types of capacitors with values ranging from 1 m f to 10 m f located close to the ltc1734l will provide adequate input bypassing. however, caution must be exercisedwhen using multilayer ceramic capacitors. because of the self resonant and high q characteristics of some types of ceramic capacitors, high voltage transients can be gener- ated under some start-up conditions, such as connecting the charger input to a hot power source. to prevent these transients from exceeding the absolute maximum voltage rating, several ohms of resistance can be added in series with the ceramic input capacitor. internal protection internal protection is provided to prevent excessive drivepin currents (i dshrt ) and excessive self-heating of the ltc1734l during a fault condition. the faults can begenerated from a shorted drive pin or from excessive drive pin current to the base of the external pnp transistor when it? in deep saturation from a very low v ce . this protection is not designed to prevent overheat- ing of the external pass transistor. indirectly though, self-heating of the pnp thermally conducting to the ltc1734l downloaded from: http:///
12 ltc1734l 1734lf ? linear technology corporation 2001 lt/tp 0802 2k ? printed in the usa part number description comments lt 1510-5 500khz constant-current/constant-voltage battery charger up to 1a charge current for li-ion, nicd, nimh or lead-acid batteries lt1571-1/lt1571-2 200khz/500khz constant-current/constant-voltage battery up to 1.5a charge current for 1-, 2- or multiple cell li-ion batteries, lt1571-5 charger family preset and adjustable battery voltages, c /10 charge detection ltc1729 li-ion battery charger termination controller can be used with ltc battery chargers to provide charge termina- tion, preset voltages, c /10 charge detection and timer functions ltc1730 li-ion battery pulse charger minimizes heat dissipation, no blocking diode required, limits maximum current for safety ltc1731 linear constant-current/constant-voltage charger controller simple charger uses external fet. features preset voltages, c /10 charge detection and programmable timer ltc1733 standalone, monolithic linear li-ion battery charger c /10 charge detection and programmable timer, 1.25a, 4.2v single-cell, thermal feedback loop ltc1734 200ma to 700ma li-ion linear charger in thinsot pin compatible with the ltc1734l, higher charge current lt1769 200khz constant-current/constant-voltage battery charger up to 2a charge current for li-ion, nicd, nimh or lead-acid batteries with input current limit related parts linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com applicatio n s i n for m atio n wu u u u package descriptio and resulting in the ic? junction temperature to rise above 150 c, thus cutting off the base current to the pnp transistor. this action will limit the transistor junctiontemperature to a temperature well above 150 c. the temperature depends on how well the ic and pnp arethermally connected and on the transistor? q ja . see the external pnp transistor section for information on pro-tecting the transistor from overheating. s6 package 6-lead plastic tsot-23 (reference ltc dwg # 05-08-1636) 1.50 ?1.75 (note 4) 2.80 bsc 0.30 ?0.45 6 plcs (note 3) datum ? 0.09 ?0.20 (note 3) s6 tsot-23 0302 2.90 bsc (note 4) 0.95 bsc 1.90 bsc 0.80 ?0.90 1.00 max 0.01 ?0.10 0.20 bsc 0.30 ?0.50 ref pin one id note:1. dimensions are in millimeters 2. drawing not to scale 3. dimensions are inclusive of plating 4. dimensions are exclusive of mold flash and metal burr 5. mold flash shall not exceed 0.254mm6. jedec package reference is mo-193 3.85 max 0.62 max 0.95 ref recommended solder pad layout per ipc calculator 1.4 min 2.62 ref 1.22 ref downloaded from: http:///

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