LTC4089/LTC4089-5
14
40895fc
then no external components are necessary. However,
if more conductance is needed, an external P-channel
MOSFET can be added from BAT to OUT. The GATE pin of
the LTC4089/LTC4089-5 drives the gate of the PFET for
automatic ideal diode control. The source of the external
MOSFET should be connected to OUT and the drain should
be connected to BAT. In order to help protect the external
MOSFET in over-current situations, it should be placed in
close thermal contact to the LTC4089/LTC4089-5.
Battery Charger
The battery charger circuits of the LTC4089/LTC4089-5
are designed for charging single cell lithium-ion batter-
ies. Featuring an internal P-channel power MOSFET, the
charger uses a constant-current/constant-voltage charge
algorithm with programmable current and a program-
mable timer for charge termination. Charge current can be
programmed up to 1.2A. The fi nal fl oat voltage accuracy
is ±0.8% typical. No blocking diode or sense resistor is
required when powering either the IN or the HVIN pins.
The CHRG open-drain status output provides information
regarding the charging status of the LTC4089/LTC4089-5
at all times. An NTC input provides the option of charge
qualifi cation using battery temperature.
An internal thermal limit reduces the programmed charge
current if the die temperature attempts to rise above a
preset value of approximately 105°C. This feature protects
the LTC4089/LTC4089-5 from excessive temperature, and
allows the user to push the limits of the power handling
capability of a given circuit board without risk of dam-
aging the LTC4089/LTC4089-5. Another benefi t of the
LTC4089/LTC4089-5 thermal limit is that charge current
can be set according to typical, not worst-case, ambient
temperatures for a given application with the assurance
that the charger will automatically reduce the current in
worst-case conditions.
The charge cycle begins when the voltage at the OUT
pin rises above the battery voltage and the battery volt-
age is below the recharge threshold. No charge current
actually fl ows until the OUT voltage is 100mV above
the BAT voltage. At the beginning of the charge cycle, if
the battery voltage is below 2.8V, the charger goes into
trickle charge mode to bring the cell voltage up to a safe
level for charging. The charger goes into the fast charge
constant-current mode once the voltage on the BAT pin
rises above 2.8V. In constant current mode, the charge
current is set by R
PROG
. When the battery approaches the
fi nal fl oat voltage, the charge current begins to decrease
as the LTC4089/LTC4089-5 switches to constant-voltage
mode. When the charge current drops below 10% of the
programmed charge current while in constant-voltage
mode the CHRG pin assumes a high impedance state.
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 fi nal fl oat potential, the internal bandgap
reference, voltage amplifi er 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:
II
V
R
CHG PROG
PROG
PROG
==•, •,50 000 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
OPERATION