Datasheet

LTC4081
13
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 fi lter 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 coeffi cient (NTC) thermistor close to the
battery pack. The NTC circuitry is shown in Figure 2.
To use this feature, connect the NTC thermistor, R
NTC
, be-
tween the NTC pin and ground and a resistor, R
NOM
, from
the NTC pin to V
CC
. 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 NTHS0603NO1N1002J
thermistor). The LTC4081 goes into hold mode when the
value of the NTC thermistor drops to 0.53 times the value
of R
NOM
, which corresponds to approximately 40°C, and
when the value of the NTC thermistor increases to 3.26
times the value of R
NOM
, 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
fi nished 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 V
IH
. 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 effi ciency
whereas the Burst Mode operation offers higher effi ciency
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
U
4081 F02
R
NOM
R
NTC
V
CC
–
+
–
+
–
+
TOO COLD
TOO HOT
NTC_ENABLE
0.76 • V
CC
0.35 • V
CC
0.016 • V
CC
6
NTC
T
Figure 2. NTC Circuit Information