LTC1731-8.2 데이터 시트보기 (PDF) - Linear Technology

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LTC1731-8.2

Lithium-Ion Linear Battery Charger Controller

Linear Technology 

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OPERATIO

The LTC1731-8.2/LTC1731-8.4 are linear battery charger

controllers. The charge current is programmed by the

combination of a program resistor (RPROG) from the

PROG pin to ground and a sense resistor (RSENSE) be-

tween the VCC and SENSE pins. RPROG sets a program

current through an internal trimmed 800Ω resistor that

creates a voltage drop from VCC to the input of the current

amplifier (CA). The current amplifier servos the gate of the

external P-channel MOSFET to force the same voltage

drop across RSENSE which sets the charge current. When

the potential at the BAT pin approaches the preset float

voltage, the voltage amplifier (VA) will start sinking cur-

rent which shrinks the voltage drop across RSENSE, thus

reducing the charge current.

Charging begins when the potential at VCC pin rises above

the UVLO level and a program resistor is connected from

the PROG pin to ground. At the beginning of the charge

cycle, if the battery voltage is below 4.95V, the charger

goes into trickle charge mode. The trickle charge current

is 10% of the full-scale current. If the cell voltage stays low

for one quarter of the total charge time, the charge

sequence will terminate.

The charger goes into the fast charge constant-current

mode after the voltage on the BAT pin rises above 4.95V.

In constant-current mode, the charge current is set by the

combination of RSENSE and RPROG.

LTC1731-8.2/LTC1731-8.4

When the battery approaches the final float voltage, the

charge current will begin to decrease. When the current

drops to 10% of the full-scale charge current, an internal

comparator will turn off the N-channel MOSFET at the

CHRG pin and connect a weak current source to ground to

indicate a near end-of-charge (C/10) condition.

An external capacitor on the TIMER pin sets the total

charge time. After a time-out occurs, the charge cycle is

terminated and the CHRG pin is forced to a high imped-

ance state. To restart the charge cycle, simply remove the

input voltage and reapply it, or float the PROG pin momen-

tarily.

For batteries like lithium-ion that require an accurate final

float potential, the internal 2.457V reference, voltage

amplifier and the resistor divider provide regulation with

±1% (max) accuracy. For NiMH and NiCd batteries, the

LTC1731-8.2/LTC1731-8.4 can be turned into a current

source by pulling the TIMER pin to VCC. When in the

constant-current only mode, the voltage amplifier, timer

and the trickle charge function are all disabled.

The charger can be shut down by floating the PROG pin

(ICC ≈ 1mA). An internal current source will pull it high and

clamp at 3.5V.

When the input voltage is not present, the charger goes

into a sleep mode, dropping ICC to 15µA. This greatly

reduces the current drain on the battery and increases the

standby time.

sn1731 17318fs

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