LTC4055 Linear Technology, LTC4055 Datasheet - Page 16

• Manufacturer Part Number: LTC4055
• Description: USB Power Controller and Li-Ion Linear Charger
• Manufacturer: Linear Technology

OPERATIO
LTC4055
where V
total resistance from the PROG pin to ground.
For example, if typical 500mA charge current is required,
calculate:
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:
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
16
R
t
TIMER
PROG
PROG
(
Hours
500
is the PROG pin voltage and R
1
V
mA
)
U
C
•
TIMER
50 000 100
,
0 1
•
.
R
PROG
F
•
100
k
•
3
k
Hours
PROG
is the
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, R
the NTC pin and ground and a resistor, R
NTC pin to V
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, R
times the value of R
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 R
resistance is R
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.
NTC
HOT
COLD
. R
, of the NTC thermistor drops to 0.41
NOM
. For a Vishay NTHS0603N02N1002J
NOM
should be a 1% resistor with a value
or approximately 4.1k, which
NOM
NTC
, from the
, between
NOM
. This
4055p