LTC1732-8.4 데이터 시트보기 (PDF) - Linear Technology

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LTC1732-8.4

Lithium-Ion Linear Battery Charger Controller

Linear Technology 

LTC1732-8.4

APPLICATIONS INFORMATION

Charger Conditions

The charger is off when any of the following conditions exist:

the VCC pin is less than 8.2V, the dropout voltage (VCC –

VBAT) is less than 54mV, or the PROG pin is floating. The

DRV pin will be pulled to VCC and the internal resistor di-

vider is disconnected 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 8.2V. To prevent oscillation around

VCC = 8.2V, 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 below 4.9V, the charger goes into trickle mode.

The charge current drops to 10% of the full-scale current.

If the low voltage persists for one quarter of the total

charge time, the battery is considered defective, the charge

cycle is terminated and the CHRG pin output is forced to

a high impedance state.

Shutdown

The LTC1732-8.4 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 thresh-

old voltage. The DRV pin is pulled up to VCC turning off the

external P-channel MOSFET. The internal timer is reset in

the shutdown mode.

Programming Charge Current

The formula for the battery charge current (see Block

Diagram) is:

IBAT = (IPROG)(800Ω/RSENSE)

= (2.457V/RPROG)(800Ω/RSENSE) or

RPROG = (2.457V/IBAT)(800Ω/RSENSE)

where RPROG is the total resistance from the PROG pin to

ground.

For example, if 0.5A 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 stability over temperature and time, 1% resistors

are recommended. The closest 1% resistor value is 19.6k.

Programming the Timer

The programmable timer terminates the charge cycle.

Typically, when charging at a 1C rate, a discharged Li-Ion

battery will become fully charged in 3 hours. For lower

charge current rates, extend the timer accordingly.The

length of the timer is programmed by an external capaci-

tor at the TIMER pin. The total charge time is:

Time (Hours) = (3 Hours) • (CTIMER/0.1µF) or

CTIMER = 0.1µF • Time (Hours)/3 (Hours)

The timer starts when an input voltage greater than 8.2V

is applied and the program resistor is connected to ground.

After a time-out occurs, the CHRG output will go into a

high impedance state to indicate that charging has stopped.

Connecting the TIMER pin to VCC disables the timer and

also puts the charger into a constant-current mode. To

only disable the timer function, short the TIMER pin to

GND.

CHRG Status Output Pin

When a charge cycle starts, the CHRG pin is pulled to

ground by an internal N-channel MOSFET that can drive an

LED. When the battery current drops to 10% of the full-

scale current (C/10), the N-channel MOSFET is turned off

and a weak 35µA current source to ground is connected to

the CHRG pin. After a time-out occurs, the pin will go into

a high impedance state. By using two different value pull-

up resistors, a microprocessor can detect three states

from this pin (charging, C/10 and stop charging). See

Figure 1 and Table 1.

17328f

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