LTC4088 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC4088' PDF : 24 Pages View PDF
LTC4088
Operation
Battery Charger
The LTC4088 includes a constant-current/constant-voltage
battery charger with automatic recharge, automatic ter-
mination by safety timer, low voltage trickle charging,
bad cell detection and thermistor sensor input for out of
temperature charge pausing.
When a battery charge cycle begins, the battery charger
first determines if the battery is deeply discharged. If the
battery voltage is below VTRKL, typically 2.85V, an automatic
trickle charge feature sets the battery charge current to
10% of the programmed value. If the low voltage persists
for more than 1/2 hour, the battery charger automatically
terminates and indicates, via the CHRG pin, that the battery
was unresponsive.
Once the battery voltage is above VTRKL, the charger be-
gins charging in full power constant-current mode. The
current delivered to the battery will try to reach 1031V/
RPROG. Depending on available input power and external
load conditions, the battery charger may or may not be
able to charge at the full programmed rate. The external
load will always be prioritized over the battery charge
current. The USB current limit programming will always
be observed and only additional power will be available to
charge the battery. When system loads are light, battery
charge current will be maximized.
Charge Termination
The battery charger has a built-in safety timer. Once the
voltage on the battery reaches the pre-programmed float
voltage of 4.200V, the charger will regulate the battery
voltage there and the charge current will decrease naturally.
Once the charger detects that the battery has reached
4.200V, the 4-hour safety timer is started. After the safety
timer expires, charging of the battery will discontinue and
no more current will be delivered.
Automatic Recharge
Once the battery charger terminates, it will remain off
drawing only microamperes of current from the battery.
If the portable product remains in this state long enough,
the battery will eventually self discharge. To ensure that the
battery is always topped off, a charge cycle will automati-
cally begin when the battery voltage falls below VRECHRG
(typically 4.1V). In the event that the safety timer is running
when the battery voltage falls below VRECHRG, it will reset
back to zero. To prevent brief excursions below VRECHRG
from resetting the safety timer, the battery voltage must be
below VRECHRG for more than 1.5ms. The charge cycle and
safety timer will also restart if the VBUS UVLO cycles low
and then high (e.g., VBUS is removed and then replaced)
or if the charger is momentarily disabled using the D2 pin.
Charge Current
The charge current is programmed using a single resistor
from PROG to ground. 1/1031th of the battery charge cur-
rent is delivered to PROG, which will attempt to servo to
1.000V. Thus, the battery charge current will try to reach
1031 times the current in the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
RPROG
=
1031V
ICHG
,
ICHG
=
1031V
RPROG
In either the constant-current or constant-voltage charging
modes, the voltage at the PROG pin will be proportional
to the actual charge current delivered to the battery. The
charge current can be determined at any time by monitoring
the PROG pin voltage and using the following equation:
IBAT
=
VPROG
RPROG
• 1031
In many cases, the actual battery charge current, IBAT,
will be lower than the programmed current, ICHG, due
to limited input power available and prioritization to the
system load drawn from VOUT.
Charge Status Indication
The CHRG pin indicates the status of the battery charger.
Four possible states are represented by CHRG which
include charging, not charging (or float charge current
less than programmed end of charge indication current),
unresponsive battery and battery temperature out of range.
The signal at the CHRG pin can be easily recognized
as one of the above four states by either a human or a
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