A Miniature, Low Dropout Battery Charger
`for Lithium-Ion Batteries – Design Note 239
`James Herr
`
`and a sense resistor (RSENSE), between the VCC and
`SENSE pins. RPROG sets a program current through an
`internal, trimmed 800Ω resistor, setting up a voltage
`drop from VCC to the input of the current amplifi er (CA).
`The current amplifi er controls the gate of an external
`P-channel MOSFET (Q1) to force an equal voltage drop
`across RSENSE, which in turn, sets the charge current.
`When the potential at the BAT pin approaches the preset
`fl oat voltage, the voltage amplifi er starts sinking cur-
`rent, which decreases the required voltage drop across
`RSENSE, thereby reducing the charge current.
`Charging begins when the potential at the VCC pin rises
`above 4.1V. At the beginning of the charge cycle, if the
`battery voltage is below 2.457V, the charger goes into
`trickle charge mode. The trickle charge current is 10%
`of the full-scale current. If the battery voltage stays low
`for one quarter of the total programmed charge time,
`the charge sequence is terminated.
`The charger goes into the fast charge, constant-current
`mode after the voltage on the BAT pin rises above
`2.457V. In constant-current mode, the charge current
`is set by the combination of RSENSE and RPROG. When
`the battery approaches the fi nal fl oat voltage, the volt-
`age loop takes control and the charge current begins
`L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
`of Linear Technology Corporation. All other trademarks are the property of their
`respective owners.
`
`VIN
`5V TO 12V
`
`MBRM120T3
`
`RSENSE
`0.2Ω
`1%
`
`Q1
`Si9430DY
`IBAT = 500mA
`
`RPROG*
`19.6k
`1%
`
`+
`
`Li-ION
`CELL
`
`1μF
`
`10μF
`
`DN239 F01
`
`8
`
`6 1 5
`
`7
`VCC
`SENSE
`DRV
`CHRG
`LTC1731-4.2
`TIMER
`BAT
`PROG
`GND
`4
`
`2 3
`
`1k
`
`CTIMER
`0.1μF
`
`*SHUTDOWN INVOKED BY FLOATING THE PROG PIN
`Figure 1. Single Cell 500mA Li-Ion Battery Charger
`
`INTRODUCTION
`Lithium-ion (Li-Ion) batteries are becoming the power
`source of choice for today’s small handheld electronic
`devices due to their light weight and high energy den-
`sity. However, there are a number of important issues
`associated with the charging of these batteries. As an
`example, if they are overcharged, they can be potentially
`hazardous to the user.
`The LTC®1731 is a constant-current/constant-voltage
`linear charger controller for single cell lithium-ion bat-
`teries. Its output accuracy of ±1% (max) over the –40°C
`to 85°C range prevents the possibility of overcharging.
`The output fl oat potential is internally set to either 4.1V
`or 4.2V, without the use of an expensive external 0.1%
`resistor divider. Furthermore, the charging current is
`user programmable with ±7% accuracy.
`At the beginning of the charging cycle, if the battery
`voltage is below 2.457V, the LTC1731 will precharge the
`battery with only 10% of the full-scale current to avoid
`stressing the depleted battery. Once the battery volt-
`age reaches 2.457V, normal charging can commence.
`Charging is terminated by a user-programmed timer.
`After this timer has completed its cycle, the charging
`can be restarted by removing and then reapplying the
`input voltage source, or by shutting down the part mo-
`mentarily. A built-in end-of-charge (C/10) comparator
`indicates that the charging current has dropped to 10%
`of the full-scale current. The output of this comparator
`can also be used to stop battery charging before the
`timer completes its cycle.
`The LTC1731 is available in the 8-pin MSOP and SO
`packages.
`OPERATION AND CIRCUIT DESCRIPTION
`Figure 1 shows a detailed schematic of a 500mA single
`cell Li-Ion battery charger using the LTC1731-4.2. The
`charge current is programmed by the combination of a
`program resistor (RPROG), from the PROG pin to ground,
`
`09/00/239_conv
`
`Fundamental Ex 2009-1
`ZTE et al. v Fundamental
`IPR2018-00110
`
`

`

`to decrease. When the current drops to 10% of the
`full-scale charge current, an internal comparator turns
`off the pull-down N-channel MOSFET at the CHRG
`pin and connects a weak current source to ground to
`indicate an end-of-charge (C/10) condition.
`An external capacitor on the TIMER pin (CTIMER) sets the
`total charging time. Once the timer cycle completes, the
`charging is terminated immediately and the CHRG pin is
`forced to a high impedance state. To restart the charge
`cycle, simply remove the input supply and then reapply
`it, or alternatively, fl oat the PROG pin momentarily.
`For batteries such as lithium-ion that require accurate
`fi nal fl oat potential, the internal 2.457V reference, volt-
`age amplifi er and the resistor divider provide regulation
`with better than ±1% accuracy. For NiMH and NiCd bat-
`teries, the LTC1731 can be turned into a current source
`by simply connecting the TIMER pin to VCC. When in
`the constant-current only mode, the voltage amplifi er,
`timer and the trickle charge function are all disabled.
`When the input voltage is not present, the charger goes
`into a sleep mode, dropping ICC to 7μA. This greatly
`reduces the current drain on the battery and increases
`the standby time. The charger can always be shut down
`by fl oating the PROG pin. An internal current source
`will pull this pin’s voltage high and clamp it at 3.5V.
`PROGRAMMING CHARGE CURRENT
`The formula for the battery charge current is:
`IBAT = (IPROG) • (800Ω/RSENSE)
`
`= (2.457V/RPROG) • (800Ω/RSENSE)
`
`R1
`1k
`
`D3
`
`2
`
`3
`
`C1
`0.1μF
`
`1μF
`CER
`
`7
`VCC
`
`8
`
`6
`
`1
`
`5
`
`SENSE
`DRV
`
`CHRG
`
`LTC1731-4.2
`
`TIMER
`
`BAT
`
`PROG
`
`GND
`4
`
`where RPROG is the total resistance from the PROG
`pin to ground.
`For example, if a 500mA charge current is needed,
`select a value for RSENSE that will drop 100mV at the
`maximum charge current. RSENSE = 0.1V/0.5A = 0.2Ω,
`then calculate:
` RPROG = (2.457V/500mA) • (800Ω/0.2Ω) = 19.656k
`For best accuracy over temperature and time, 1%
`resistors are recommended. The closest 1% resistor
`value is 19.6k.
`TYPICAL APPLICATION
`1.5A Single Cell Battery Charger
`The LTC1731 can also be connected as a switcher-based
`battery charger for higher charging current applications
`(see Figure 2). As in the linear charger, the charge current
`is set by R3 and R5. The CHRG pin output will indicate
`an end-of-charge (C/10) condition when the average
`current drops down to 10% of the full-scale value. A
`100μF bypass capacitor is required at the BAT pin to
`keep the ripple voltage low.
`CONCLUSION
`The LTC1731 makes a very compact, low parts count
`and low cost multiple battery chemistry charger. The
`onboard programmable timer provides charge termina-
`tion without interfacing to a microprocessor.
`
`C2
`22μF
`CER
`
`D1
`
`VIN
`5V TO 6V
`
`D2
`
`R3
`0.082Ω
`1%
`
`Q1
`
`L1
`
`4
`
`25
`
`C4
`0.47μF
`
`R2
`5Ω
`
`3
`
`TPS2829DBVR
`
`R5
`18.2k
`1%
`
`+
`
`+
`
`1-CELL
`Li-ION
`BATTERY
`
`C3
`100μF
`
`C1: AVX0603ZC104KAT1A
`D1, D2: MOTOROLA MBRS130LT3
`(800) 441-2447
`Q1: SILICONIX Si3443DV
`(800) 554-5565
`L1: SUMIDA CDRH6D38-220NC
`(847) 956-0666
`Figure 2. Single Cell 1.5A Li-Ion Battery Charger
`
`DN239 F02
`
`Data Sheet Download
`www.linear.com
`
`Linear Technology Corporation
`1630 McCarthy Blvd., Milpitas, CA 95035-7417
`(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
`
`For applications help,
`call (408) 432-1900
`
`dn239f_conv LT/TP 0900 340K • PRINTED IN THE USA
`
`© LINEAR TECHNOLOGY CORPORATION 2000
`
`Fundamental Ex 2009-2
`ZTE et al. v Fundamental
`IPR2018-00110
`
`