LTC3558
14
3558f
APPLICATIONS INFORMATION
when V
CC
falls to within V
DUVLO
(typically 50mV) of the
BAT voltage.
Suspend Mode
The battery charger can also be disabled by pulling the
SUSP pin above 1.2V. In suspend mode, the battery
drain current is reduced to 1.5μA and the input current is
reduced to 8.5μA.
Charge Cycle Overview
When a battery charge cycle begins, the battery charger
fi rst determines if the battery is deeply discharged. If the
battery voltage is below V
TRKL
, typically 2.9V, an automatic
trickle charge feature sets the battery charge current to
10% of the full-scale value.
Once the battery voltage is above 2.9V, the battery charger
begins charging in constant-current mode. When the
battery voltage approaches the 4.2V required to maintain
a full charge, otherwise known as the fl oat voltage, the
charge current begins to decrease as the battery charger
switches into constant-voltage mode.
Trickle Charge and Defective Battery Detection
Any time the battery voltage is below V
TRKL
, the charger
goes into trickle charge mode and reduces the charge
current to 10% of the full-scale current. If the battery
voltage remains below V
TRKL
for more than 1/2 hour, the
charger latches the bad-battery state, automatically termi-
nates, and indicates via the
CHRG
pin that the battery was
unresponsive. If for any reason the battery voltage rises
above V
TRKL
, the charger will resume charging. Since the
charger has latched the bad-battery state, if the battery
voltage then falls below V
TRKL
again but without rising past
V
RECHRG
fi rst, the charger will immediately assume that
the battery is defective. To reset the charger (i.e., when
the dead battery is replaced with a new battery), simply
remove the input voltage and reapply it or put the part in
and out of suspend mode.
Charge Termination
The battery charger has a built-in safety timer that sets
the total charge time for 4 hours. Once the battery voltage
rises above V
RECHRG
(typically 4.105V) and the charger
enters constant-voltage mode, the 4-hour timer is started.
After the safety timer expires, charging of the battery will
discontinue and no more current will be delivered.
Automatic Recharge
After 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 V
RECHRG
(typically 4.105V). In the event that the safety timer is
running when the battery voltage falls below V
RECHRG
, it
will reset back to zero. To prevent brief excursions below
V
RECHRG
from resetting the safety timer, the battery voltage
must be below V
RECHRG
for more than 1.7ms. The charge
cycle and safety timer will also restart if the V
CC
UVLO or
DUVLO cycles low and then high (e.g., V
CC
is removed
and then replaced) or the charger enters and then exits
suspend mode.
Programming Charge Current
The PROG pin serves both as a charge current program
pin, and as a charge current monitor pin. By design, the
PROG pin current is 1/800th of the battery charge current.
Therefore, connecting a resistor from PROG to ground
programs the charge current while measuring the PROG pin
voltage allows the user to calculate the charge current.
Full-scale charge current is defi ned as 100% of the con-
stant-current mode charge current programmed by the
PROG resistor. In constant-current mode, the PROG pin
servos to 1V if HPWR is high, which corresponds to charg-
ing at the full-scale charge current, or 200mV if HPWR
is low, which corresponds to charging at 20% of the full-
scale charge current. Thus, the full-scale charge current
and desired program resistor for a given full-scale charge
current are calculated using the following equations:
I
V
R
R
V
I
CHG
PROG
PROG
CHG
=
=
800
800
