LTC4055EUF datasheet(16/24 Pages) LINER

16 / 24 page

LTC4055

4055p

OPERATIO

where VPROG is the PROG pin voltage and RPROG is the

total resistance from the PROG pin to ground.

For example, if typical 500mA charge current is required,

calculate:

PROG ==

500

50 000

100

•,

For best stability over temperature and time, 1% metal film

resistors are recommended. Under trickle charge condi-

tions, this current is reduced to 10% of the full-scale value.

THE CHARGE TIMER

The programmable charge timer is used to terminate the

charge cycle. The timer duration is programmed by an

external capacitor at the TIMER pin and is also a function

of the resistance on PROG. Typically the charge time is:

Hours

TIMER

PROG

()

••

.•

0 1

100

The timer starts when an input voltage greater than the

undervoltage lockout threshold level is applied, or when

leaving shutdown and the voltage on the battery is less

than the recharge threshold. At power up or exiting shut-

down with the battery voltage less than the recharge

threshold, the charge time is a full cycle. If the battery is

greater than the recharge threshold, the timer will not start

and charging is prevented. If after power-up the battery

voltage drops below the recharge threshold, or if after a

charge cycle the battery voltage is still below the recharge

threshold, the charge time is set to one half of a full cycle.

The LTC4055 has a feature that extends charge time

automatically. Charge time is extended if the charge cur-

rent in constant-current mode is reduced due to load

current, undervoltage charge current limiting or thermal

regulation. This change in charge time is inversely propor-

tional to the change in charge current. As the LTC4055

approaches constant-voltage mode the charge current

begins to drop. This change in charge current is part of the

normal charging operation of the part and should not

affect the timer duration. Therefore, the LTC4055 detects

that the change in charge current is due to voltage mode,

and increases the timer period back to its programmed

operating period.

Once a time-out occurs and the voltage on the battery is

greater than the recharge threshold, the charge current

stops, and the CHRG output assumes a high impedance

state to indicate that the charging has stopped.

Connecting the TIMER pin to ground disables the battery

charger.

CHRG STATUS OUTPUT PIN

When the charge cycle starts, the CHRG pin is pulled to

ground by an internal N-channel MOSFET capable of

driving an LED. After a time-out occurs, the pin assumes

a high impedance state.

NTC Thermistor

The battery temperature is measured by placing a negative

temperature coefficient (NTC) thermistor close to the

battery pack. The NTC circuitry is shown in Figure 3. 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 LTC4055 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 LTC4055

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

approximately 0

°C. The hot and cold comparators each

have approximately 3

°C of hysteresis to prevent oscilla-

tion about the trip point. Grounding the NTC pin disables

the NTC function.

LTC4055EUF Datasheet(PDF) 16 Page - Linear Technology

LTC4055EUF Datasheet(HTML) 16 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL