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LTC4081

4081f

Timer and Recharge

The LTC4081’s battery charger has an internal charge

termination timer that starts when the input voltage is

greater than the undervoltage lockout threshold and at

least 82mV above BAT, and the battery charger is leaving

shutdown.

At power-up or when exiting shutdown, the charge time

is set to 4.5 hours. Once the charge cycle terminates, the

battery charger continuously monitors the BAT pin voltage

using a comparator with a 2ms ﬁlter time. When the aver-

age battery voltage falls below 4.1V (which corresponds

to 80%-90% battery capacity), a new charge cycle is initi-

ated and a 2.25 hour timer begins. This ensures that the

battery is kept at, or near, a fully charged condition and

eliminates the need for periodic charge cycle initiations.

The ⎯C⎯H⎯R⎯G output assumes a strong pulldown state dur-

ing recharge cycles until C/10 is reached or the recharge

cycle terminates.

Battery Temperature Monitoring via NTC

The battery temperature is measured by placing a nega-

tive temperature coefﬁcient (NTC) thermistor close to the

battery pack. The NTC circuitry is shown in Figure 2.

To use this feature, connect the NTC thermistor, RNTC, be-

tween the NTC pin and ground and a resistor, RNOM, from

the NTC pin to VCC. 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 NTHS0603NO1N1002J

thermistor). The LTC4081 goes into hold mode when the

value of the NTC thermistor drops to 0.53 times the value

of RNOM, which corresponds to approximately 40°C, and

when the value of the NTC thermistor increases to 3.26

times the value of RNOM, which corresponds to approxi-

mately 0°C. Hold mode freezes the timer and stops the

charge cycle until the thermistor indicates a return to a

valid temperature. For a Vishay NTHS0603NO1N1002J

thermistor, this value is 32.6k 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.

When the charger is in Hold mode (battery temperature

is either too hot or too cold) the ⎯C⎯H⎯R⎯G pin pulses in a

2Hz, 25% duty cycle frequency unless the charge task is

ﬁnished or the battery is assumed to be defective. If the

NTC pin is grounded, the NTC function will be disabled.

SWITCHING REGULATOR OPERATION:

The switching buck regulator in the LTC4081 can be turned

on by pulling the EN_BUCK pin above VIH. It has two user-

selectable modes of operation: constant-frequency (PWM)

mode and Burst Mode Operation. The constant-frequency

mode operation offers low noise at the expense of efﬁciency

whereas the Burst Mode operation offers higher efﬁciency

at light loads at the cost of increased noise, higher output

voltage ripple, and less output current. A detailed descrip-

tion of different operating modes and different aspects of

operation follow. Operations can best be understood by

referring to the Block Diagram.

OPERATIO

4081 F02

RNOM

RNTC

VCC

–

TOO COLD

TOO HOT

NTC_ENABLE

0.76 • VCC

0.35 • VCC

0.016 • VCC

NTC

Figure 2. NTC Circuit Information

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