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LTC4085

4085fc

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 volt-

age 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.Whenthebatteryapproachestheﬁnalﬂoatvoltage,

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.

OPERATION

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