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LTC4050

4050f

APPLICATIONS INFORMATION

Figure 1. Microprocessor Interface

Charger Conditions

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

the voltage at the VCC pin is below 4V, the voltage at the VCC

pin is greater than 4V but is less than 54mV above VBAT,

the PROG pin is floating, the timer has timed out or the

thermistor temperature is outside the acceptable range. In

this condition, the DRV pin is pulled to VCC and the internal

resistor divider 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 the undervoltage lockout threshold of 4V. To

prevent oscillation around VCC = 4V, 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 2.49V), the charger goes into trickle

mode. In this mode, the charge current is reduced to 10%

of the full-scale current. If the low cell voltage persists for

one quarter of the total charge time, the battery is consid-

ered defective, the charge cycle is terminated and the

CHRG pin output becomes high impedance.

Shutdown

The LTC4050 can be forced into shutdown by floating the

PROG pin and allowing the internal 2.3

µA current source

to pull the pin above the 3.6V shutdown threshold voltage.

In shutdown, the DRV pin is pulled up to VCC, turning off

the external P-channel MOSFET and resetting the internal

timer.

Programming Charge Current

The formula for the battery charge current (see Block

Diagram) is:

IBAT = (IPROG)(800Ω/RSENSE)

= (2.47V/RPROG)(800Ω/RSENSE) or

RPROG = (2.47V/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 = (2.47V/500mA)(800Ω/0.2Ω) = 19.76k

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 time 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 4V is

applied and the program resistor is connected to ground.

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

impedance to indicate that charging has stopped. To dis-

able the timer function, short the TIMER pin to GND.

400k

CHRG

4050 F01

VCC

LTC4050

V +

OUT

µPROCESSOR

VDD

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