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LTC4066/LTC4066-1

4066fc

APPLICATIONS INFORMATION

Programming Charge Current

The formula for the battery charge current is:

CHG

PROG

()

•,

50 000

where VPROG is the PROG pin voltage and RPROG is the

total resistance from the PROG pin to ground. Keep in

mind that when the LTC4066/LTC4066-1 are powered

from the IN pin, the programmed input current limit takes

precedence over the charge current. In such a scenario,

the charge current cannot exceed the programmed input

current limit.

For example, if typical 500mA charge current is required,

calculate:

PROG

= ⎛

⎝⎜

⎞

⎠⎟

500

50 000

100

•,

For best stability over temperature and time, 1% metal

ﬁlm resistors are recommended. Under trickle charge

conditions, this current is reduced to 10% of the full-

scale value.

Monitoring Charge Current

The ISTAT and POL pins provide a means for monitoring

the BAT pin current. The ISTAT pin sources a current equal

to one-thousandth of the absolute value of the current

ﬂowing in the BAT pin. The POL pin indicates the polarity

of the BAT pin current. When current is ﬂowing from OUT

to BAT (i.e., charging), the POL pin pulls to ground. When

current is ﬂowing from BAT to OUT (ideal diode), the POL

pin assumes a high impedance. If a resistor, RISTAT, is

placed from the ISTAT pin to ground, then the formula for

BAT current is:

BAT

ISTAT

• 1000

where VISTAT is the ISTAT pin voltage and RISTAT is the total

resistance from the ISTAT pin to ground. These pins enable

a true gas gauge function to be performed on the battery

with an external ADC and integrator. See Gas Gauge for

more information.

The Charge Timer

The programmable charge timer is used to terminate the

charge cycle. The timer duration is programmed by an

external capacitor at the TIMER pin. The charge time is

typically:

Hours

TIMER

PROG

()

••

.•

0 1

100

The timer starts when an input voltage greater than the

undervoltage lockout threshold level is applied or when

leaving shutdown and the voltage on the battery is less than

the recharge threshold. At power-up or exiting shutdown

with the battery voltage less than the recharge threshold,

the charge time is a full cycle. If the battery is greater than

the recharge threshold, the timer will not start and charging

is prevented. If after power-up the battery voltage drops

below the recharge threshold, or if after a charge cycle

the battery voltage is still below the recharge threshold,

the charge time is set to one-half of a full cycle.

The LTC4066/LTC4066-1 have a feature that extends charge

time automatically. Charge time is extended if the charge

current in constant-current mode is reduced due to load

current or thermal regulation. This change in charge time

is inversely proportional to the change in charge current.

As the LTC4066/LTC4066-1 approach constant-voltage

mode the charge current begins to drop. This change in

charge current is due to normal charging operation and

does not affect the timer duration.

LTC4066 Datasheet(PDF) 18 Page - Linear Technology

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