LTC1731-4.1 Просмотр технического описания (PDF) - Linear Technology

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LTC1731-4.1

Single Cell Lithium-Ion Linear Battery Charger Controllers

Linear Technology 

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OPERATIO

The LTC1731 is a linear battery charger controller for

single cell lithium-ion batteries. The charge current is

programmed by the combination of a program resistor

(RPROG) from the PROG pin to ground and a sense resistor

(RSENSE) between the VCC and SENSE pins. RPROG sets a

program current through an internal trimmed 800Ω resis-

tor setting up 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

current 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 2.457V, 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 be 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.

LTC1731-4.1/LTC1731-4.2

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 pull-down N-channel MOSFET

at the CHRG pin and connect a weak current source to

ground to indicate an end-of-charge (C/10) condition.

An external capacitor on the TIMER pin sets the total

charge time. After a time-out occurs, the charging will be

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

momentarily.

For batteries like lithium-ion that require 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 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 7µA. This

greatly reduces the current drain on the battery and

increases the standby time.

APPLICATIONS INFORMATION

Charge Termination

The charger is off when any of the following conditions

exist: the voltage at the VCC pin is below 4.1V, the voltage

at the VCC pin is higher than 4.1V but is less than 54mV

above VBAT, or the PROG pin is floating. The DRV pin will

be pulled to VCC and the internal resistor divider is discon-

nected to reduce the current drain on the battery.

Undervoltage Lockout (UVLO)

An internal undervoltage lockout circuit monitors the input

voltage and keeps the charger in shutdown mode until VCC

rises above 4.1V. To prevent oscillation around

VCC = 4.1V, the UVLO circuit has built-in hysteresis.

Trickle Charge and Defective Battery Detection

At the beginning of the charging sequence, if the battery

voltage is low (below 2.457V) the charger goes into trickle

charge mode. The charge current is set to 10% of the full-

scale current. If the low cell voltage persists for one

quarter of the total charging time, the battery is considered

defective, charging will be terminated and the CHRG pin

output is forced to a high impedance state.

Shutdown

The LTC1731 can be forced into shutdown by floating the

PROG pin and allowing the internal 2.5µA current source

to pull the pin above the 2.457V shutdown threshold

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