LTC1731-4.2 Linear Technology, LTC1731-4.2 Datasheet - Page 6

• Manufacturer Part Number: LTC1731-4.2
• Description: Lithium-Ion Linear Battery Charger Controller
• Manufacturer: Linear Technology

APPLICATIONS
LTC1731-4.2
threshold voltage. The DRV pin will then be pulled up to
V
internal timer is reset in the shutdown mode.
Programming Charge Current
The formula for the battery charge current (see Block
Diagram) is:
where R
ground.
For example, if 0.5A charge current is needed, select a
value for R
charge current. R
For best stability over temperature and time, 1% resistors
are recommended. The closest 1% resistor value is 19.6k.
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 battery 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
6
CC
I
R
Time = (3 Hours)(C
BAT
PROG
and turn off the external P-channel MOSFET. The
= (I
= (2.457V/R
PROG
= (2.457V/500mA)(800 /0.2 ) = 19.656k
PROG
SENSE
is the total resistance from the PROG pin to
)(800 /R
SENSE
that will drop 100mV at the maximum
U
PROG
TIMER
= 0.1V/0.5A = 0.2 , then calculate:
INFORMATION
)(800 /R
U
SENSE
/0.1 F)
)
SENSE
W
)
CC
disables the
U
from this pin (charging, C/10 and stop charging). See
Figure 1.
End of Charge (C/10)
The LTC1731-4.2 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
comparator 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 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 LTC1731-4.2 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 too much
charging 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 LTC1731-4.2 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
LTC1731-4.2
Figure 1. Microprocessor Interface
V
V
CC
+
CHRG
7
2
SENSE
100k
1k
), it represents 1.67% of
I/O
I/O
CC
PROCESSOR
. This feature allows
CC
. This is particu-
1731 F01
PROG
=