Datasheet
LTC4089-3
14
40893f
For more information www.linear.com/4089-3
OUT is sufficient to keep a transition from input power
to battery power from causing significant output voltage
droop. The ideal diode consists of a precision amplifier that
enables a large P-channel MOSFET transistor whenever
the voltage at OUT is approximately 20mV (V
FWD
) below
the voltage at BAT. The resistance of the internal ideal
diode is approximately 200m
. If this is sufficient for the
application 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 LTC4089-3 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 LTC4089-3.
Battery Charger
The battery charger circuits of the LTC4089-3 are designed
for charging single-cell lithium-ion batteries. Featuring
an internal P-channel power MOSFET, the charger uses a
constant-current/constant-voltage charge algorithm with
programmable current and a programmable timer for
charge termination. Charge current can be programmed
up to 1.2A. The final float 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 LTC4089-3 at all times. An NTC
input provides the option of charge qualification 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 LTC4089-3 from excessive temperature, and allows
the user to push the limits of the power handling capabil-
ity of a given circuit board without risk of damaging the
LTC4089-3. Another benefit of the LTC4089-3 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 flows 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
final float voltage, the charge current begins to decrease as
the LTC4089-3 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.
OPERATION
Figure 3. LTC4089-3 Versus Schottky
Diode Forward Voltage Drop
CONSTANT
I
0N
CONSTANT
R
0N
CONSTANT
V
0N
V
FWD
I
MAX
FORWARD VOLTAGE (V)
40893 F03
I
FWD
0
CURRENT (A)
SLOPE: 1/R
DIO(ON)
SLOPE: 1/R
FWD
SCHOTTKY
DIODE
LTC4089