Datasheet

LTC4089/LTC4089-5
15
40895fc
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:
R
V
mA
k
PROG
==
1
500
50 000 100•,
For best stability over temperature and time, 1% metal fi lm
resistors are recommended. Under trickle charge conditions,
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:
t hours
C R hours
μF k
TIMER
TIMER PROG
()
••
.•
=
3
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
R
CLPROG
= 2k, R
PROG
= 100k and C
TIMER
= 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 fi ve until the
charger reaches constant voltage mode (i.e. V
BAT
= 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 fl ow 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:
I
V
R
V
R
DETECT
PROG PROG
==
01
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 fi rst 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.
OPERATION