LTC4085 datasheet(18/24 Pages) LINER

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LTC4085

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For example, if the full charge current is programmed

to 500mA with a 100k PROG resistor the CHRG pin will

change state at a battery charge current of 50mA.

Note: The end-of-charge (EOC) comparator that moni-

tors the charge current latches its decision. Therefore,

the ﬁrst time the charge current drops below 10% of the

programmed full charge current while in constant-volt-

age mode will toggle CHRG to a high impedance state.

If, for some reason, the charge current rises back above

the threshold the CHRG pin will not resume the strong

pull-down state. The EOC latch can be reset by a recharge

cycle (i.e. VBAT drops below the recharge threshold) or

toggling the input power to the part.

Current Limit Undervoltage Lockout

An internal undervoltage lockout circuit monitors the

input voltage and disables the input current limit circuits

until VIN rises above the undervoltage lockout threshold.

The current limit UVLO circuit has a built-in hysteresis of

125mV. Furthermore, to protect against reverse current in

the power MOSFET, the current limit UVLO circuit disables

the current limit (i.e. forces the input power path to a high

impedance state) if VOUT exceeds VIN. If the current limit

UVLO comparator is tripped, the current limit circuits will

not come out of shutdown until VOUT falls 50mV below

the VIN voltage.

Charger Undervoltage Lockout

An internal undervoltage lockout circuit monitors the VOUT

voltage and disables the battery charger circuits until VOUT

rises above the undervoltage lockout threshold. The battery

charger UVLO circuit has a built-in hysteresis of 125mV.

Furthermore, to protect against reverse current in the power

MOSFET, the charger UVLO circuit keeps the charger shut

down if VBATexceedsVOUT.IfthechargerUVLOcomparator

is tripped, the charger circuits will not come out of shut

down until VOUT exceeds VBAT by 50mV.

Suspend

The LTC4085 can be put in suspend mode by forcing the

SUSP pin greater than 1V. In suspend mode the ideal diode

function from BAT to OUT is kept alive. If power is applied

to the OUT pin externally (i.e., a wall adapter is present)

then charging will be unaffected. Current drawn from the

IN pin is reduced to 50μA. Suspend mode is intended to

comply with the USB power speciﬁcation mode of the

same name.

NTC Thermistor

The battery temperature is measured by placing a negative

temperature coefﬁcient (NTC) thermistor close to the bat-

tery pack. The NTC circuitry is shown in Figure 4. To use

this feature, connect the NTC thermistor (RNTC) between

the NTC pin and ground and a resistor (RNOM) from the

NTC pin to VNTC. RNOM should be a 1% resistor with a

value equal to the value of the chosen NTC thermistor at

25°C (this value is 10k for a Vishay NTHS0603N02N1002J

thermistor). The LTC4085 goes into hold mode when the

resistance (RHOT) of the NTC thermistor drops to 0.41

times the value of RNOM or approximately 4.1k, which

should be at 50°C. The hold mode freezes the timer and

stops the charge cycle until the thermistor indicates a

return to a valid temperature. As the temperature drops,

the resistance of the NTC thermistor rises. The LTC4085 is

designed to go into hold mode when the value of the NTC

thermistor increases to 2.82 times the value of RNOM. This

resistance is RCOLD. For a Vishay NTHS0603N02N1002J

thermistor, this value is 28.2k which corresponds to ap-

proximately 0°C. The hot and cold comparators each have

approximately 3°C of hysteresis to prevent oscillation

about the trip point. Grounding the NTC pin can disable

the NTC function.

OPERATION

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