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LTC4089/LTC4089-5

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when the LTC4089/LTC4089-5 is powered from the IN pin,

the programmed input current limit takes precedent 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

resistorsarerecommended.Undertricklechargeconditions,

this current is reduced to 10% of the full-scale value.

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

µF

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 LTC4089/LTC4089-5 has 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 LTC4089/LTC4089-5 approaches 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.

Consider, for example, a USB charge condition where

RCLPROG = 2k, RPROG = 100k and CTIMER = 0.1µF. This

corresponds to a three hour charge cycle. However, if the

HPWR input is set to a logic low, then the input current

limit will be reduced from 500mA to 100mA. With no ad-

ditional system load, this means the charge current will

be reduced to 100mA. Therefore, the termination timer

will automatically slow down by a factor of ﬁve until the

charger reaches constant voltage mode (i.e. VBAT = 4.2V)

or HPWR is returned to a logic high. The charge cycle is

automatically lengthened to account for the reduced charge

current. The exact time of the charge cycle will depend on

how long the charger remains in constant-current mode

and/or how long the HPWR pin remains a logic low.

Once a time-out occurs and the voltage on the battery is

greater than the recharge threshold, the charge current

stops, and the CHRG output assumes a high impedance

state if it has not already done so.

Connecting the TIMER pin to ground disables the battery

charger.

CHRG Status Output Pin

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

ground by an internal N-channel MOSFET capable of driv-

ing an LED. When the charge current drops below 10%

of the programmed full charge current while in constant

voltage mode, the pin assumes a high impedance state,

but charge current continues to ﬂow until the charge

time elapses. If this state is not reached before the end

of the programmable charge time, the pin will assume a

high impedance state when a time-out occurs. The CHRG

current detection threshold can be calculated by the fol-

lowing equation:

DETECT

PROG

50 000

5000

•,

For example, if the full charge current is programmed

to 500mA with a 100k PROG resistor the CHRG pin will

change state at a battery charge current of 50mA.

Note: The end-of-charge (EOC) comparator that moni-

tors the charge current latches its decision. Therefore,

the ﬁrst time the charge current drops below 10% of the

programmed full charge current while in constant volt-

age mode, it will toggle CHRG to a high impedance state.

OPERATIO

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