LTC4089-3 Datasheet, PDF (15/24 Pages) Linear Technology – USB Power Manager with High Voltage Switching Charger

LTC4089-3
OPERATION
An external capacitor on the TIMER pin sets the total
minimum charge time. When this time elapses the
charge cycle terminates and the CHRG pin assumes a
high impedance state, if it has not already done so. While
charging in constant-current mode, if the charge current
is decreased by thermal regulation or in order to maintain
the programmed input current limit the charge time is
automatically increased. In other words, the charge time
is extended inversely proportional to the actual charge
current delivered to the battery. For Li-Ion and similar
batteries that require accurate final float potential, the
internal bandgap reference, voltage amplifier and the
resistor divider provide regulation with ±0.8% accuracy.
Trickle Charge and Defective Battery Detection
At the beginning of a charge cycle, if the battery volt-
age is low (below 2.8V) the charger goes into trickle
charge reducing the charge current to 10% of the full-
scale current. If the low battery voltage persists for one
quarter of the total charge time, the battery is assumed
to be defective, the charge cycle is terminated and the
CHRG pin output assumes a high impedance state. If
for any reason the battery voltage rises above ~2.8V the
charge cycle will be restarted. To restart the charge cycle
(i.e., when the dead battery is replaced with a discharged
battery), simply remove the input voltage and reapply it
or cycle the TIMER pin to 0V.
Programming Charge Current
The formula for the battery charge current is:
I_{C H G}
= I_PROG
• 50,000
=
V_{P R O G}
R_PROG
• 50,000
where V_PROG is the PROG pin voltage and R_PROG is the
total resistance from the PROG pin to ground. Keep in
mind that when the LTC4089-3 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_PROG
=
1V
500mA
•
50,000
=
100k
For best stability over temperature and time, 1% metal film
resistors are recommended. Under trickle charge condi-
tions, 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 _{T I M E R} (h o u rs)
=
C
TIMER ^{• R}PROG ^{• 3hours}
0.1µF • 100k
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 LTC4089-3 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 LTC4089-3 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.
For more information www.linear.com/4089-3
40893f
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