LTC4085EDE-3 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC4085EDE-3
Linear Technology
'LTC4085EDE-3' PDF : 24 Pages View PDF
LTC4085-3/LTC4085-4
OPERATION
Battery Charger
The battery charger circuits of the LTC4085 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 1.5A. The final float voltage accuracy is ±0.8% typi-
cal. No blocking diode or sense resistor is required when
powering the IN pin. The CHRG open-drain status output
provides information regarding the charging status of the
LTC4085 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 LTC4085 from excessive temperature, and allows the
user to push the limits of the power handling capability of a
given circuit board without risk of damaging the LTC4085.
Another benefit of the LTC4085 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 output UVLO level and the battery voltage
is below the recharge threshold. No charge current actually
flows until the OUT voltage is greater than the output UVLO
level and 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 RPROG. When
the battery approaches the final float voltage, the charge
current begins to decrease as the LTC4085 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 charge current de-
livered 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 voltage
is low (below 2.8V) the charger goes into trickle charge
reducing the charge current to 10% of the full-scale cur-
rent. 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.
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