LTC4055/LTC4055-1
16
4055fb
OPERATION
where V
PROG
is the PROG pin voltage and R
PROG
is the
total resistance from the PROG pin to ground.
For example, if typical 485mA charge current is required,
calculate:
R
V
mA
k
PROG
==
1
485
48 500 100•,
For best stability over temperature and time, 1% metal
fi lm resistors are recommended. Under trickle-charge
conditions, 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:
t Hours
C R Hours
Fk
TIMER
TIMER PROG
()
••
.•
=
μ
3
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 shutdown
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/LTC4055-1 have a feature that extends
charge time automatically. Charge time is extended if
the charge current in constant-current mode is reduced
due to load current, undervoltage charge current limit-
ing or thermal regulation. This change in charge time is
inversely proportional to the change in charge current. As
the LTC4055/LTC4055-1 approach 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/LTC4055-1 detect that the change in charge
current is due to voltage mode, and increase 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 coeffi cient (NTC) thermistor close to the
battery pack. The NTC circuitry is shown in Figure 3. To use
this feature, connect the NTC thermistor, R
NTC
, between
the NTC pin and ground and a resistor, R
NOM
, from the
NTC pin to V
NTC
. R
NOM
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/LTC4055-1 go into hold mode
when the resistance, R
HOT
, of the NTC thermistor drops
to 0.41 times the value of R
NOM
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/
LTC4055-1 are designed to go into hold mode when the
value of the NTC thermistor increases to 2.82 times the
value of R
NOM
. This resistance is R
COLD
. 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 oscillation about the trip point. Grounding the
NTC pin disables the NTC function.
