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LTC1734 Lithium-Ion Linear Battery Charger in ThinSOT
FEATURES
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DESCRIPTIO
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Low Profile (1mm) ThinSOTTM Package No Blocking Diode Required No Sense Resistor Required 1% Accurate Preset Voltages: 4.1V or 4.2V Charge Current Monitor Output for Charge Termination Programmable Charge Current: 200mA to 700mA Automatic Sleep Mode with Input Supply Removal Manual Shutdown Negligible Battery Drain Current in Shutdown Undervoltage Lockout Self Protection for Overcurrent/Overtemperature
The LTC(R)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 program 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 nickelcadmium (NiCd) and nickel-metal-hydride (NiMH) batteries using external termination.
, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation.
APPLICATIO S
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Cellular Telephones Handheld Computers Digital Cameras Charging Docks and Cradles Low Cost and Small Size Chargers Programmable Current Sources
TYPICAL APPLICATIO
PROG Pin Indicates Charge Status
5V VBAT (V)
300mA Li-Ion Battery Charger
VIN 5V 1F 3 2 4 RPROG 5k VCC ISENSE 1
4V
LTC1734 6 GND DRIVE PROG BAT 5
3V
FMMT549 IBAT = 300mA 10F
VPROG (V) 2V 1.5V 1V
CONSTANT CURRENT VPROG
+
SINGLE Li-Ion BATTERY
1734 TA01
0V CHARGING BEGINS
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VBAT CONSTANT VOLTAGE CHARGING COMPLETE
1734 TA01b
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1
LTC1734
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
ORDER PART NUMBER
TOP VIEW ISENSE 1 GND 2 VCC 3 6 DRIVE 5 BAT 4 PROG
Supply Voltage (VCC) ...................................- 0.3V to 9V Input Voltage (BAT, PROG) ........ - 0.3V to (VCC + 0.3V) Output Voltage (DRIVE) .............. - 0.3V to (VCC + 0.3V) Output Current (ISENSE) ................................... - 900mA Short-Circuit Duration (DRIVE) ...................... Indefinite Junction Temperature .......................................... 125C Operating Ambient Temperature Range (Note 2) ...............................................-40C to 85C Operating Junction Temperature (Note 2) ............ 100C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
LTC1734ES6-4.1 LTC1734ES6-4.2 S6 PART MARKING LTHD LTRG
S6 PACKAGE 6-LEAD PLASTIC SOT-23
TJMAX = 125C, JA = 230C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V, GND = 0V and VBAT 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.
SYMBOL VCC Supply VCC ICC ISHDN IBMS IBSL VUVLOI VUVLOD VUVHYS VBAT IBAT1 IBAT2 VCM1 Operating Supply Range (Note 5) Quiescent VCC Pin Supply Current VCC Pin Supply Current in Manual Shutdown Battery Drain Current in Manual Shutdown (Note 3) Battery Drain Current in Sleep Mode (Note 4) Undervoltage Lockout Exit Threshold Undervoltage Lockout Entry Threshold Undervoltage Lockout Hysteresis Output Float Voltage in Constant Voltage Mode Output Full-Scale Current When Programmed for 200mA in Constant Current Mode Output Full-Scale Current When Programmed for 700mA in Constant Current Mode Current Monitor Voltage on PROG Pin VBAT = 5V, (Forces IDRIVE = IBAT = 0), IPROG = 200A,(7500 from PROG to GND) PROG Pin Open PROG Pin Open VCC = 0V VCC Increasing VCC Decreasing VCC Decreasing 4.1V Version, IBAT = 10mA, 4.55V VCC 8V 4.2V Version, IBAT = 10mA, 4.55V VCC 8V RPROG = 7500, 4.55V VCC 8V, Pass PNP Beta > 50 RPROG = 2143, 4.55V VCC 8V, Pass PNP Beta > 50 IBAT = 10% of IBAT1, RPROG = 7500, 4.55V VCC 8V, Pass PNP Beta > 50, 0C TA 85C IBAT = 10% of IBAT2, RPROG = 2143, 4.55V VCC 8V, Pass PNP Beta > 50, 0C TA 85C VDRIVE = 3.5V
q q q q q q q q q q q q
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN 4.55
TYP
MAX 8
UNITS V A A A A V V mV
670 450 -1 -1 4.45 4.30 0 0 4.56 4.41 150 4.059 4.158 155 620 0.045 4.10 4.20 200 700 0.15
1150 900 1 1 4.68 4.53
Charging Performance 4.141 4.242 240 770 0.28 V V mA mA V
VCM2
Current Monitor Voltage on PROG Pin
0.10
0.15
0.20
IDSINK
Drive Output Current
30
2
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V
mA
LTC1734
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V, GND = 0V and VBAT 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.
SYMBOL VMSDT VMSHYS IPROGPU Protection IDSHRT Drive Output Short-Circuit Current Limit VDRIVE = VCC
q
ELECTRICAL CHARACTERISTICS
PARAMETER Manual Shutdown Threshold Manual Shutdown Hysteresis Programming Pin Pull-Up Current
CONDITIONS VPROG Increasing VPROG Decreasing from VMSDT VPROG = 2.5V
q
MIN 2.05 -6 35
TYP 2.15 90 -3 65
MAX 2.25 - 1.5 130
UNITS V mV A mA
Charger Manual Control
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 0C to 70C ambient temperature range and 0C to 100C junction temperature range. Specifications over the - 40C to 85C 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 (VBAT) and the base is biased at 5V (VCC).
Note 4: Assumes that the external PNP pass transistor has negligible B-E reverse-leakage current when the emitter is biased at 0V (VCC) and the base is biased at 4.2V (VBAT). Note 5: The 4.68V maximum undervoltage lockout (UVLO) exit threshold must first be exceeded before the minimum VCC specification applies. Short duration drops below the minimum VCC specification of several microseconds or less are ignored by the UVLO. If manual shutdown is entered, then VCC must be higher than the 4.68V maximum UVLO threshold before manual shutdown can be exited. When operating near the minimum VCC, a suitable PNP transistor with a low saturation voltage must be used.
TYPICAL PERFOR A CE CHARACTERISTICS
Float Voltage vs Temperature and Supply Voltage
4.21 IBAT = 10mA PNP = FCX589 4.2V OPTION 4.201
FLOAT VOLTAGE (V)
FLOAT VOLTAGE (V)
4.20 VCC = 8V VCC = 4.55V
4.200
IBAT1 (mA)
4.19 -50 -25
50 25 0 75 TEMPERATURE (C)
UW
100
1734 G01
Float Voltage vs IBAT
VCC = 5V TA = 25C PNP = FCX589 4.2V OPTION RPROG = 2150 210
IBAT1 vs Temperature and Supply Voltage
RPROG = 7.5k PNP = FCX589
200 VCC = 4.55V AND 8V
125
4.199 0 100 200 300 400 IBAT (mA) 500 600 700
190 -50 -25
50 25 0 75 TEMPERATURE (C)
100
125
1734 G02
1734 G03
3
LTC1734 TYPICAL PERFOR A CE CHARACTERISTICS
IBAT2 vs Temperature and Supply Voltage
740 RPROG = 2.15k PNP = FCX589
IBAT1 (mA)
IBAT2 (mA)
700 VCC = 4.55V AND 8V
200
BAT PIN MUST BE DISCONNECTED AND GROUNDED TO FORCE CC MODE IN THIS REGION
IBAT2 (mA)
660 -50 -25
50 25 0 75 TEMPERATURE (C)
Program Pin Pull-Up Current vs Temperature and Supply Voltage
3.6 3.5 VCC = 8V
IPROGPU (A)
IPROGPU (A) 3.6
VPROG = 2.5V
3.4
3.4 3.3 VCC = 4.55V 3.2 3.1 3.0 -50 -25
VPROG (V)
50 25 75 0 TEMPERATURE (C)
Program Pin Voltage vs Charge Current (700mA)
1.6 VCC = 5V T = 25C 1.4 A RPROG = 2.15k PNP = FCX589 1.2 160
VPROG (mV)
0.8 0.6 0.4 0.2 0 0 100 200 300 400 IBAT2 (mA) 500 600 700 LIMITS AT 6mV DUE TO PROGRAMMING PIN PULL-UP CURRENT (IPROGPU)
150
VCC = 8V
VPROG (mV)
VPROG (V)
1.0
4
UW
100
1734 G04
IBAT1 vs VBAT
210 VCC = 5V TA = 25C RPROG = 7.5k PNP = FCX589 750
IBAT2 vs VBAT
VCC = 5V TA = 25C RPROG = 2.15k PNP = FCX589
700
BAT PIN MUST BE DISCONNECTED AND GROUNDED TO FORCE CC MODE IN THIS REGION
125
190
0
1
3 2 VBAT (V)
4
5
1734 G05
650
0
1
3 2 VBAT (V)
4
5
1734 G06
Program Pin Pull-Up Current vs VPROG
VCC = 8V TA = 25C
1.6 1.4 1.2
Program Pin Voltage vs Charge Current (200mA)
VCC = 5V TA = 25C RPROG = 7.5k PNP = FCX589
3.2
1.0 0.8 0.6 0.4 LIMITS AT 25mV DUE TO PROGRAMMING PIN PULL-UP CURRENT (IPROGPU) 0 50 100 IBAT1 (mA) 150 200
1734 F09
3.0
2.8
0.2
2.6
100
125
2
3
4
5 6 VPROG (V)
7
8
1635 G08
0
1734 G07
Program Pin Voltage for IBAT1/10 vs Temperature and Supply Voltage
RPROG = 7.5k PNP = FCX589 160
Program Pin Voltage for IBAT2/10 vs Temperature and Supply Voltage
RPROG = 2.15k PNP = FCX589
VCC = 8V 150 VCC = 4.55V
VCC = 4.55V
140 -50 -25
50 25 0 75 TEMPERATURE (C)
100
125
140 -50 -25
50 25 0 75 TEMPERATURE (C)
100
125
1734 G10
1734 G11
1734 G12
LTC1734
PIN FUNCTIONS
ISENSE (Pin 1): Sense Node for Charge Current. Current from VCC passes through the internal current sense resistor and reappears at ISENSE 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. VCC (Pin 3): Positive Input Supply Voltage. This pin supplies power to the internal control circuitry and external PNP transistor through the internal current sense resistor. This pin should be bypassed to ground with a capacitor in the range of 1F to 10F. 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 (RPROG) 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 (IBAT = 1500/RPROG). This pin also allows for the charge current to be monitored. The voltage on this pin is proportional 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 instability. 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 34A 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 5F 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.
BLOCK DIAGRA
VOLTAGE REFERENCE 2.5V
UVLO
2.15V
+
SHUTDOWN
-
3A
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VIN 1F 3 VCC
IBAT/1000
60
IBAT
0.06 1 SHUTDOWN
ISENSE
REF
-
A3
+
OUTPUT DRIVER TEMPERATURE AND CURRENT LIMITING 6
DRIVE
SHUTDOWN
IBAT BAT 5 10F SINGLE Li-Ion CELL
C1 1.5V
+
A2 A1
+ -
2.5V
-
SHUTDOWN 4 2
1734 BD
PROG RPROG
GND
5
LTC1734
OPERATIO
The LTC1734 is a linear battery charger controller. Operation can best be understood by referring to the Block Diagram. Charging begins when VCC rises above the UVLO (Undervoltage Lockout) threshold VUVLOI and an external current programming resistor is connected between the PROG pin and ground. When charging, the collector of the external PNP provides the charge current. The PNP's emitter current flows through the ISENSE pin and through the internal 0.06 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 resistor to appear across the internal 60 resistor. The scale factor of 1000:1 in resistor values will cause the FET's 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 limiting the charge current. This is the constant current mode. The constant charge current is 1000 * (1.5V/RPROG). 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
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
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4.2V (2.5V at amplifier A1's 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 maximum current set by the current programming resistor. The battery charge current is 1000 * (VPROG/RPROG) 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 VCC is applied, the charger can be manually shut down by floating the otherwise grounded end of RPROG. An internal 3A 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 excessive 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 VCC removed the external PNP's base is connected to the battery by the charger. In manual shutdown the base is connected to VCC by the charger. 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 rising above 5V, allowing the precision constant voltage circuit time to respond.
LTC1734
APPLICATIONS INFORMATION
Manual Shutdown Floating the program resistor allows an internal 3A current source (IPROGPU) to pull the PROG pin above the 2.15V shutdown threshold (VMSDT), thus shutting down the charger. In this mode, the LTC1734 continues to draw some current from the supply (ISHDN), but only a negligible leakage current is delivered to the battery (IBMS). Shutdown can also be accomplished by pulling the otherwise grounded end of the program resistor to a voltage greater than 2.25V (VMSDTMax). Charging will cease above 1.5V, but the internal battery voltage resistor divider will draw about 34A from the battery until shutdown is entered. Figure 1 illustrates a microcontroller configuration 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 impedance to ground. The programming resistor will prevent potentially damaging currents if the PROG pin is forced above VCC. Under this condition VCC may float, be loaded down by other circuitry or be shorted to ground. If VCC is not shorted to ground the current through the resistor will pull VCC up somewhat. Another method is to directly switch the PROG pin to a voltage source when shutdown is desired (Caution: pulling the PROG below 1.5V with VCC applied will cause excessive and uncontrolled charge currents). The voltage source must be capable of sourcing the resulting current through the program resistor. This has the advantage of not adding any error to the program resistor during normal operation. The voltage on the PROG pin must be greater than 2.25V (VMSDT(MAX)) to ensure
RPROG OPEN DRAIN OR TOTEM POLE OUTPUT C ADC INPUT
1734 F01
PROG LTC1734
Figure 1. Interfacing with a Microcontroller
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entering shutdown, but no more than 0.3V above VCC to prevent damaging the LTC1734 from excessive PROG pin current. An exception is if VCC is allowed to float with no other circuitry loading VCC 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 VCC, 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 VCC 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 recommended 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 (IBSL) is a negligible leakage current, allowing the battery to remain 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 (VUVLOI) 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 (< 1A).
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LTC1734
APPLICATIONS INFORMATION
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 700A 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 programmed is approximately 50mA, which requires a program resistor of 30k. Limiting capacitive loading on the program pin becomes more important when high value program resistors are used. In addition, the current
VIN 5V 1F OPTIONAL FILTER CHARGE CURRENT MONITOR (FILTERED) 1k PIN 4 0.1F TO 0.5F CHARGE CURRENT MONITOR (UNFILTERED) 3
3k Q1 2N7002
CONTROL 1
Figure 2. Logic Control Programming of Output Current to 0mA, 200mA, 500mA or 700mA
VIN 5V 1F 3 2 4 3k Q1 2N7002 1
VCC
LTC1734 6 GND DRIVE PROG BAT 5
7.5k Q2 2N7002
CONTROL 1
CONTROL 2
Figure 3. Programmable Current Source with Output Current of 0mA, 200mA, 500mA or 700mA
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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 IDAC 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 300s 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.
VCC ISENSE 1 CHARGE CURRENT CONTROL 1 CONTROL 2 0 LOW LOW 200mA LOW HIGH 500mA HIGH LOW 700mA HIGH HIGH SINGLE Li-Ion BATTERY
1734 F02
LTC1734 2 6 GND DRIVE 4 PROG BAT 5
FZT549 IBAT 10F
7.5k Q2 2N7002
CONTROL 2
ISENSE
FZT549* ILOAD LOAD
1734 F03
CURRENT CONTROL 1 CONTROL 2 0 LOW LOW 200mA LOW HIGH 500mA HIGH LOW 700mA HIGH HIGH
*OBSERVE MAXIMUM TEMPERATURE
LTC1734
APPLICATIONS INFORMATION
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 * (VPROG/RPROG) 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. See Figure 1 for an example. The minimum PROG pin current is about 3A (IPROGPU). 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/IBAT(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 inadvertent 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 dissipation 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 (VCESAT) becomes important. The VCESAT must be less than the minimum supply voltage minus the maximum voltage drop across the internal sense resistor and bond wires (0.1) and battery float voltage. If the PNP transistor 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's 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 operating VCE is: VCE(MIN)(V) = 4.75 - (0.5)(0.1) - 4.2 = 0.5V The actual battery charge current (IBAT) 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 (IC/IB), IBAT can be calculated as follows: IBAT(A) = 1000 * IPROG[/( + 1)] If = 50, then IBAT is 2% low. If desired, the 2% loss can be compensated for by increasing IPROG by 2%. Another important factor to consider when choosing the PNP pass transistor is the power handling capability. The transistor's 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: PD(MAX)(W) = IBAT (VDD(MAX) - VBAT(MIN)) VDD(MAX) is the maximum supply voltage and VBAT(MIN) is the minimum battery voltage when discharged.
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LTC1734
APPLICATIONS INFORMATION
Table 1. PNP Pass Transistor Selection Guide
Maximum PD (W) Mounted on Board at TA = 25C 0.5 0.625 1 1.1 1 to 2 2 2 0.75 1 2 10 (TC = 25C) Package Style SOT-23 SOT-23 SOT-89 SOT-23-6 SOT-89 SOT-223 SOT-223 FTR ATV SOT-89 TO-252 ZETEX Part Number FMMT549 FMMT720 FCX589 or BCX69 ZXT10P12DE6 FCX717 FZT589 BCP69 or FZT549 2SB822 2SB1443 2SA1797 2SB1182 Low VCESAT Low VCESAT Low VCESAT Low VCESAT, High Beta Very Low VCESAT, High Beta, Small Very Low VCESAT, High Beta Low VCESAT ROHM Part Number Comments Low VCESAT Very Low VCESAT, High Beta
Once the maximum power dissipation and VCE(MIN) are known, Table 1 can be used as a guide in selecting some PNPs to consider. In the table, very low VCESAT is less than 0.25V, low VCESAT is 0.25V to 0.5V and the others are 0.5V to 0.8V all depending on the current. See the manufacturer's 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 85C to 145C making it behave somewhat like a thermostat switch. For example, the model PTH9C16TBA471Q thermistor is 470 at 25C, but abruptly increase its resistance to 4.7k at 125C. Below 125C, the device exhibits a small negative TC. The 470 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 125C, 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
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constant voltage mode, a capacitor of at least 4.7F is usually required from BAT to ground. The battery and interconnecting wires appear inductive at high frequencies, and since these are in the feedback loop, this capacitance may be necessary to compensate for the inductance. This capacitor need not exceed 100F 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 4.7F to 22F and an ESR of 0.5 to 1.5. 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 to 1.5 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, capacitance 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 capacitance is greater than that given by the following equation:
CMAX(pF) = 400k RPROG
LTC1734
APPLICATIONS INFORMATION
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 (RPROG = 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 programmed 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
10k TO SCOPE RPROG 3k 2.5V 0V f = 1kHz
1734 F04
PROG
BAT
+
Li-Ion*
LTC1734
*FULLY CHARGED CELL
Figure 4. Setup for AC Stability Testing
5V 0V
PULSE GENERATOR
2V PROG PIN (20pF ON PIN) 1V 0V 2V PROG PIN (200pF ON PIN) 1V 0V SHUT DELAY DOWN CONSTANT CURRENT CONSTANT VOLTAGE
HORIZONTAL SCALE: 100s/DIV
Figure 5. Stability Waveforms
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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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 VCC 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.
* VIN VCC LTC1734
1734 F06
*DRAIN-BULK DIODE OF FET
Figure 6. Low Loss Reverse Voltage Protection
VCC Bypass Capacitor Many types of capacitors with values ranging from 1F to 10F 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 generated 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 (IDSHRT) 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 it's in deep saturation from too low a VCE. 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
6 TO 20
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LTC1734
APPLICATIONS INFORMATION
resulting in the IC's junction temperature to rise above 150C, thus cutting off the PNP's base current. This action will limit the PNP's junction temperature to some temperature well above 150C. The temperature depends on how well the IC and PNP are thermally connected and on the transistor's JA. See the External PNP Transistor section for information on protecting the transistor from overheating.
PACKAGE DESCRIPTIO
.20 (.008) DATUM `A' A A2 2.60 - 3.00 1.50 - 1.75 (.102 - .118) (.059 - .069) (NOTE 3) 1.90 (.074) REF SOT-23 (Original) A A1 A2 L .90 - 1.45 (.035 - .057) .00 - 0.15 (.00 - .006) .90 - 1.30 (.035 - .051) .35 - .55 (.014 - .021) SOT-23 (ThinSOT) 1.00 MAX (.039 MAX) .01 - .10 (.0004 - .004) .80 - .90 (.031 - .035) .30 - .50 REF (.012 - .019 REF) .95 (.037) REF PIN ONE ID A1
L NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES)
.09 - .20 (.004 - .008) (NOTE 2)
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
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sn1734 1734fs LT/TP 0801 2K * PRINTED IN THE USA
LT1769
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
www.linear.com
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S6 Package 6-Lead Plastic SOT-23
(LTC DWG # 05-08-1634) (LTC DWG # 05-08-1636)
2.80 - 3.10 (.110 - .118) (NOTE 3)
.25 - .50 (.010 - .020) (6PLCS, NOTE 2)
S6 SOT-23 0401
COMMENTS Up to 1A Charge Current for Li-Ion, NiCd, NiMH or Lead-Acid Batteries Up to 1.5A Charge Current for 1-, 2- or Multiple Cell Li-Ion Batteries, Preset and Adjustable Battery Voltages, C/10 Charge Detection Can be Used with LTC Battery Chargers to Provide Charge Termination, Preset Voltages, C/10 Charge Detection and Timer Functions Minimizes Heat Dissipation, No Blocking Diode Required, Limits Maximum Current for Safety
(c) LINEAR TECHNOLOGY CORPORATION 2001

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