LTC1731-4.1 Linear Technology, LTC1731-4.1 Datasheet - Page 6

• Manufacturer Part Number: LTC1731-4.1
• Description: Lithium-Ion Linear Battery Charger Controller
• Manufacturer: Linear Technology

APPLICATIONS
LTC1731-4.1
Programming the Timer
The programmable timer is used to terminate the charge.
The length of the timer is programmed by an external
capacitor at the TIMER pin. The total charge time is:
The timer starts when the input voltage greater than 4.1V
is applied and the program resistor is connected to ground.
After a time-out occurs, the CHRG output will turn into a
high impedance state to indicate that the charging has
stopped. Connecting the TIMER pin to V
timer and also puts the charger into a constant-current
mode. To only disable the timer function, short the TIMER
pin to GND.
CHRG Status Output Pin
When the charge cycle starts, the CHRG pin is pulled down
to ground by an internal N-channel MOSFET that can drive
an LED. When the charge current drops to 10% of the full-
scale current (C/10), the N-channel MOSFET is turned off
and a weak 100 A current source to ground is connected
to the CHRG pin. After a time-out occurs, the pin will go
into a high impedance state. By using two different value
pull-up resistors, a microprocessor can detect three states
from this pin (charging, C/10 and stop charging). See
Figure 1.
When the LTC1731 is in charge mode, the CHRG pin is
pulled down by an internal N-channel MOSFET. To detect
this mode, force the digital output pin, OUT, high and
measure the voltage at the CHRG pin. The N-channel
MOSFET will pull the pin low even with a 1k pull-up
resistor. Once the charge current drops to 10% of the full
scale current (C/10), the N-channel MOSFET is turned off
6
Time = (3 Hours)(C
LTC1731-4.1
Figure 1. Microprocessor Interface
V
V
CC
+
CHRG
7
U
2
TIMER
INFORMATION
U
100k
/0.1 F)
1k
OUT
IN
W
PROCESSOR
V
DD
CC
1731 F01
disables the
U
and a 100 A current source is connected to the CHRG pin.
By forcing the OUT pin into a high impedance state, the
current source will pull the pin low through the 100k
resistor. When the internal timer has expired, the CHRG
pin will change to high impedance state and the 100k
resistor will then pull the pin high to indicate the charging
has stopped.
End of Charge (C/10)
The LTC1731 includes a comparator to monitor the charge
current to detect an end-of-charge condition. When the
battery current falls below 10% of full scale, the compara-
tor trips and turns off the N-channel MOSFET at the CHRG
pin and switches in a 100 A current source to ground.
After an internal time delay of at least 300ms, this state is
then latched. This delay will help prevent false triggering
due to transient currents. The end-of-charge comparator
is disabled in trickle charge mode.
Gate Drive
Typically the LTC1731 controls an external P-channel
MOSFET to supply current to the battery. The DRV pin is
internally clamped to 6.5V below V
low voltage P-channel MOSFETs with gate to source
breakdown voltage rated at 8V to be used.
An external PNP transistor can also be used as the pass
transistor instead of the P-channel MOSFET. Due to the
low current gain of the current amplifier (CA), a high gain
Darlington PNP transistor is required to avoid excessive
charge current error. The gain of the current amplifier is
around 0.6 A/mV. For every 1 A of base current, a 1.6mV
of gain error shows up at the inputs of CA. With R
19.6k (100mV across R
error in charging current.
Constant-Current Only Mode
The LTC1731 can be used as a programmable current
source by forcing the TIMER pin to V
larly useful for charging NiMH or NiCd batteries. In the
constant-current only mode, the timer and voltage ampli-
fier are both disabled. An external termination method is
required to properly terminate the charge.
SENSE
), it represents 1.67% of
CC
. This feature allows
CC
. This is particu-
PROG
=