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1 ltc1734 lithium-ion linear battery charger in thinsot the ltc ? 1734 is a low cost, single cell, constant-current/ constant-voltage li-ion battery charger controller. when combined with a few external components, the sot-23 package forms a very small, low cost charger for single cell lithium-ion batteries. the ltc1734 is available in 4.1v and 4.2v versions with 1% accuracy. 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 ltc1734 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 ltc1734 from operating outside of safe limits. no external blocking diode is required. the ltc1734 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. n low profile (1mm) thinsot tm package n no blocking diode required n no sense resistor required n 1% accurate preset voltages: 4.1v or 4.2v n charge current monitor output for charge termination n programmable charge current: 200ma to 700ma n automatic sleep mode with input supply removal n manual shutdown n negligible battery drain current in shutdown n undervoltage lockout n self protection for overcurrent/overtemperature n cellular telephones n handheld computers n digital cameras n charging docks and cradles n low cost and small size chargers n programmable current sources , ltc and lt are registered trademarks of linear technology corporation. 300ma li-ion battery charger prog pin indicates charge status features descriptio u applicatio s u typical applicatio u v in 5v i bat = 300ma 1734 ta01 single li-ion battery 10 f + gnd prog drive bat 2 4 6 5 ltc1734 v cc i sense 31 1 f r prog 5k fmmt549 charging begins charging complete 1734 ta01b 5v 4v 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.
2 ltc1734 order part number (note 1) supply voltage (v cc ) ...................................C 0.3v to 9v input voltage (bat, prog) ........ C 0.3v to (v cc + 0.3v) output voltage (drive) .............. C 0.3v to (v cc + 0.3v) output current (i sense ) ................................... C 900ma short-circuit duration (drive) ...................... indefinite junction temperature .......................................... 125 c operating ambient temperature range (note 2) ...............................................C40 c to 85 c operating junction temperature (note 2) ............ 100 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c ltc1734es6-4.1 ltc1734es6-4.2 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 lthd ltrg the l 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, u nless otherwise specified. electrical characteristics i sense 1 gnd 2 v cc 3 6 drive 5 bat 4 prog top view s6 package 6-lead plastic sot-23 symbol parameter conditions min typ max units v cc supply v cc operating supply range (note 5) l 4.55 8 v i cc quiescent v cc pin supply current v bat = 5v, (forces i drive = i bat = 0), l 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 l 450 900 m a i bms battery drain current in manual shutdown prog pin open l C1 0 1 m a (note 3) i bsl battery drain current in sleep mode (note 4) v cc = 0v l C1 0 1 m a v uvloi undervoltage lockout exit threshold v cc increasing l 4.45 4.56 4.68 v v uvlod undervoltage lockout entry threshold v cc decreasing l 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 4.1v version, i bat = 10ma, 4.55v v cc 8v l 4.059 4.10 4.141 v 4.2v version, i bat = 10ma, 4.55v v cc 8v l 4.158 4.20 4.242 v i bat1 output full-scale current when programmed r prog = 7500 w , 4.55v v cc 8v, l 155 200 240 ma for 200ma in constant current mode pass pnp beta > 50 i bat2 output full-scale current when programmed r prog = 2143 w , 4.55v v cc 8v, l 620 700 770 ma for 700ma 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 , l 0.05 0.15 0.28 v 4.55v v cc 8v, pass pnp beta > 50 v cm2 current monitor voltage on prog pin i bat = 10% of i bat2 , r prog = 2143 w , l 0.10 0.15 0.20 v 4.55v v cc 8v, pass pnp beta > 50 i dsink drive output current v drive = 3.5v l 30 ma consult ltc marketing for parts specified with wider operating temperature ranges.
3 ltc1734 the l 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, u nless otherwise specified. electrical characteristics symbol parameter conditions min typ max units charger manual control v msdt manual shutdown threshold v prog increasing l 2.05 2.15 2.25 v v mshys manual shutdown hysteresis v prog decreasing from v msdt 90 mv i progpu programming pin pull-up current v prog = 2.5v C 6 C 3 C 1.5 m a protection i dshrt drive output short-circuit current limit v drive = v cc l 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 ltc1734e 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 C 40 c to 85 c operating ambient temperature range are assured by design, characterization and correlation 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 is entered, then v cc must be higher than the 4.68v maximum uvlo threshold before manual shutdown can be exited. when operating near the minimum v cc , a suitable pnp transistor with a low saturation voltage must be used. typical perfor a ce characteristics uw float voltage vs temperature and 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 4.2v option v cc = 8v v cc = 4.55v i bat (ma) 0 4.199 float voltage (v) 4.200 4.201 200 400 500 1734 g02 100 300 600 700 v cc = 5v t a = 25 c pnp = fcx589 4.2v option r prog = 2150 temperature ( c) ?0 190 i bat1 (ma) 200 210 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
4 ltc1734 typical perfor a ce characteristics uw i bat2 vs temperature and supply voltage i bat1 vs v bat temperature ( c) ?0 660 i bat2 (ma) 700 740 0 50 75 1734 g04 ?5 25 100 125 r prog = 2.15k pnp = fcx589 v cc = 4.55v and 8v v bat (v) 0 i bat1 (ma) 210 4 1734 g05 200 190 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) 750 4 1734 g06 700 650 1 2 3 5 v cc = 5v t a = 25 c r prog = 2.15k pnp = fcx589 bat pin must be disconnected and grounded to force cc mode in this region program pin pull-up current vs temperature and supply voltage program pin pull-up current vs v prog program pin voltage vs charge current (200ma) 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 1.2 200 1734 f09 0.6 0.4 0 50 100 150 0.2 1.6 1.4 v cc = 5v t a = 25 c r prog = 7.5k pnp = fcx589 limits at 25mv due to programming pin pull-up current (i progpu ) program pin voltage vs charge current (700ma) program pin voltage for i bat1 /10 vs temperature and supply voltage i bat2 (ma) 0 v prog (v) 1.4 300 1734 g10 0.8 0.4 100 200 400 0.2 0 1.6 1.2 1.0 0.6 500 600 700 v cc = 5v t a = 25 c r prog = 2.15k pnp = fcx589 limits at 6mv due to programming 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 = 2.15k pnp = fcx589 v cc = 8v v cc = 4.55v
5 ltc1734 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 single li-ion cell 6 drive 1 i sense v cc 1 f v in i bat /1000 temperature and current limiting 60 0.06 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 charge current 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 ltc1734 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. this pin supplies power to the internal control circuitry and exter- nal pnp transistor 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 a virtual 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 the constant current mode. the typical charge current will be 1000 times greater than the current through this resistor (i bat = 1500/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 this pin. 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 inductive effects of the battery and wiring. drive (pin 6): base drive output for the external pnp pass transistor. provides a controlled sink current that drives the base of the pnp. this pin has current limiting protection for the ltc1734.
6 ltc1734 4.2v (2.5v at amplifier a1s input) the amplifier will divert current away from the output driver thus limiting charge current to that which will maintain 4.2v on the battery. this is the constant voltage mode. when in the constant voltage mode, the 1000:1 current ratio is still valid and the voltage on the prog pin will indicate the charge current as a proportion of the maxi- mum current set by the current programming resistor. the battery charge current is 1000 ? (v prog /r prog ) amps. this feature allows a microcontroller with an adc to easily monitor charge 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 initiating shutdown. for charging nimh or nicd batteries, the ltc1734 can function as a constant current source by grounding the bat pin. this will prevent amplifier a1 from trying to limit charging 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 pnps base is connected to the battery by the charger. in manual shutdown the base is connected to v cc by the charger. operatio u the ltc1734 is a linear battery charger controller. opera- tion 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 current programming resistor is connected be- tween the prog pin and ground. when charging, the collector of the external pnp provides the charge current. the pnps emitter current flows through the i sense pin and through the internal 0.06 w current sense resistor. this current is close in magnitude, but slightly more than the collector current since it includes the base current. amplifier a3, along with the p-channel fet, will force the same voltage that appears across the 0.06 w resistor to appear across the internal 60 w resistor. the scale factor of 1000:1 in resistor values will cause the fets drain current to be 1/1000 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 maximum that is programmed by r prog . the prog pin current, which is 1/1000 of the charge current, 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 1000 ? (1.5v/r prog ). as the battery accepts charge, its voltage rises. when it reaches the preset float voltage of 4.2v (ltc1734-4.2 version), 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 applicatio s i for atio wu uu charging operation charging begins when an input voltage is present that exceeds the undervoltage lockout threshold (v uvloi ), a li-ion battery is connected to the charger output and a program 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 constant voltage 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 fast built-in protection circuit will prevent the bat pin from ris- ing above 5v, allowing the precision constant voltage circuit time to respond.
7 ltc1734 applicatio n s i n for m atio n wu u u manual shutdown floating 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 ltc1734 continues to draw some current from the supply (i shdn ), but only a negli- gible 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 will draw 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 will pull v cc up somewhat. another method is to directly switch the prog pin to a voltage 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 ltc1734 from excessive prog pin 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 pin shutdown 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 to implement 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. the use of an npn allows for 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 voltage source 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 for normal 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 the threshold level. in undervoltage lockout, battery drain current is very low (< 1 m a). figure 1. interfacing with a microcontroller ltc1734 prog c r prog adc input 1734 f01 open drain or totem pole output
8 ltc1734 applicatio n s i n for m atio n wu u u programming constant current when in the constant current mode, the full-scale charge current (c) is programmed using a single external resistor between the prog pin and ground. this charge current will be 1000 times greater than the current through the program resistor. the program resistor value is selected by dividing the voltage forced across the resistor (1.5v) by the desired resistor current. the ltc1734 is designed for a maximum current of approximately 700ma. this translates to a maximum prog pin current of 700 m a and a minimum program resistor of approximately 2.1k. because the prog pin is in a closed-loop signal path, the pole 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 50ma, which requires a pro- gram resistor of 30k. 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 very low current levels. if current monitoring is desired, a minimum full-scale current of 200ma is recommended. different charge currents can be programmed by various means 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 output from 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 for unless 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 200ma minimum constant current. v in 5v charge current monitor (unfiltered) charge current monitor (filtered) 7.5k q2 2n7002 control 2 fzt549 i bat 1734 f02 single li-ion battery 10 f gnd prog drive bat 2 4 6 5 ltc1734 v cc i sense 31 1 f 0.1 f to 0.5 f 1k optional filter 3k pin 4 q1 2n7002 control 1 charge current 0 200ma 500ma 700ma control 1 low low high high control 2 low high low high v in 5v 7.5k q2 2n7002 control 2 fzt549* i load *observe maximum temperature 1734 f03 gnd prog drive bat 2 4 6 5 ltc1734 v cc i sense 31 1 f 3k q1 2n7002 control 1 current 0 200ma 500ma 700ma control 1 low low high high control 2 low high low high load figure 2. logic control programming of output current to 0ma, 200ma, 500ma or 700ma figure 3. programmable current source with output current of 0ma, 200ma, 500ma or 700ma
9 ltc1734 monitoring charge current the 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 1000 ? (v prog /r prog ) amps. this feature allows a microcontrol- ler with an adc to easily monitor charge current and if desired, manually shut down the charger at the appropri- ate time. see figure 1 for an example. the minimum prog pin current is about 3 m a (i progpu ). errors in the charge current monitor voltage on the prog pin are inversely proportional to battery current and can be statistically approximated as follows: one sigma error(%) @ 1 + 0.3/i bat (a) dynamic loads on the battery will cause transients to appear 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 inadvert- ent momentary 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 ltc1734 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 voltages of 4.1v or 4.2v by grounding the bat pin. the program resistor will determine the output current. the output current range can be between approximately 50ma and 700ma, depending on the maximum power rating of the external pnp pass transistor. external pnp transistor the external pnp pass transistor must have adequate beta, low saturation voltage and sufficient power dissipa- tion capability (including any heat sinking, if required). to provide 700ma of charge current with the minimum available base drive of approximately 30ma requires a pnp beta greater than 23. if lower beta pnp transistors are used, more base current is required from the ltc1734. this can result in the output drive current limit being reached, or thermal shutdown due to excessive power dissipation. excessive beta can affect ac stability (see stability section) 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 maximum voltage drop across the internal sense resistor and bond wires (0.1 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 chargers 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 500ma with a minimum supply voltage of 4.75v, the minimum operat- ing v ce is: v ce(min) (v) = 4.75 C (0.5)(0.1) C 4.2 = 0.5v the actual battery charge current (i bat ) is slightly smaller than the expected charge current because the charger senses 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) = 1000 ? 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 the pnp pass transistor is the power handling capability. the transistors 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) C 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
10 ltc1734 once the maximum power dissipation and v ce(min) are known, table 1 can be used as a guide in selecting some pnps to consider. 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. see the manufacturers 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 pth9c 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 pth9c16tba471q thermistor 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 current programming resistor for a 700ma charger. should the thermistor reach 125 c, the charge current will drop to 238ma and inhibit any further increase in temperature. stability the ltc1734 contains two control loops: constant voltage 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 and interconnecting 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 the inductance 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 0.5 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 10 (t c = 25 c) to-252 2sb1182 low v cesat, high beta
11 ltc1734 higher charge currents require lower program resistor values which can tolerate more capacitive loading on the prog pin. maximum capacitance can be as high as 50pf for a charge current of 200ma (r prog = 7.5k). figure 4 is a simple test circuit for checking stability in both the 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 4.1v or 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 esr values, 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, where the voltage drop must be kept low, a p-channel fet as shown in figure 6 can be used. figure 5. stability waveforms ltc1734 prog li-ion* 6 to 20 *fully charged cell 10k r prog 3k to scope 1734 f04 bat 2.5v f = 1khz 0v + figure 4. setup for ac stability testing 5v 0v prog pin (20pf on pin) prog pin (200pf on pin) pulse generator 2v 1v 0v 2v shut down delay constant current horizontal scale: 100 s/div 1v 0v constant voltage v cc v in * 1734 f06 ltc1734 *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 ltc1734 will provide adequate input bypassing. however, caution must be exercised when 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 drive pin currents (i dshrt ) and excessive self-heating of the ltc1734 during a fault condition. the faults can be generated from a shorted drive pin or from excessive drive pin current to the base of the external pnp transistor when its in deep saturation from too low a v ce . this protection is not designed to prevent overheating of the external pass transistor. indirectly though, self-heating of the pnp thermally conducting to the ltc1734 and
12 ltc1734 ? linear technology corporation 2001 1734f lt/tp 0801 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 l i-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 ltc1732 linear constant-current/constant-voltage charger controller simple charger uses external fet. input power good indication features preset voltages, c /10 charge detection and program- mable timer 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 l fax: (408) 434-0507 l www.linear.com applicatio n s i n for m atio n wu u u u package descriptio resulting in the ics junction temperature to rise above 150 c, thus cutting off the pnps base current. this action will limit the pnps junction temperature to some temperature well above 150 c. the temperature depends on how well the ic and pnp are thermally connected and on the transistors q ja . see the external pnp transistor section for information on protecting the transistor from overheating. s6 package 6-lead plastic sot-23 (ltc dwg # 05-08-1634) (ltc dwg # 05-08-1636) l datum ? .09 ?.20 (.004 ?.008) (note 2) a1 s6 sot-23 0401 a a2 1.90 (.074) ref .20 (.008) 1.50 ?1.75 (.059 ?.069) (note 3) 2.60 ?3.00 (.102 ?.118) .25 ?.50 (.010 ?.020) (6plcs, note 2) 2.80 ?3.10 (.110 ?.118) (note 3) .95 (.037) ref pin one id millimeters (inches) note: 1. controlling dimension: millimeters 2. dimensions are in 3. drawing not to scale 4. dimensions are inclusive of plating 5. dimensions are exclusive of mold flash and metal burr 6. mold flash shall not exceed .254mm 7. package eiaj reference is: sc-74a (eiaj) for original jedel mo-193 for thin .90 ?1.45 (.035 ?.057) .00 ?0.15 (.00 ?.006) .90 ?1.30 (.035 ?.051) .35 ?.55 (.014 ?.021) 1.00 max (.039 max) a a1 a2 l .01 ?.10 (.0004 ?.004) .80 ?.90 (.031 ?.035) .30 ?.50 ref (.012 ?.019 ref) sot-23 (original) sot-23 (thinsot)

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