LTC1731-4.1 Datasheet, PDF (8/12 Pages) Linear Technology – Single Cell Lithium-Ion Linear Battery Charger Controllers

LTC1731-4.1/LTC1731-4.2
APPLICATIONS INFORMATION
voltage. The DRV pin will then be pulled up to V_CC and
turn off the external P-channel MOSFET. The internal
timer is reset in the shutdown mode.
Programming Charge Current
The formula for the battery charge current (see Block
Diagram) is:
I_BAT = (I_PROG)(800Ω/R_SENSE)
= (2.457V/R_PROG)(800Ω/R_SENSE)
where R_PROG is the total resistance from the PROG pin to
ground.
For example, if 0.5A charge current is needed, select a
value for R_SENSE that will drop 100mV at the maximum
charge current. R_SENSE = 0.1V/0.5A = 0.2Ω, then calculate:
R_PROG = (2.457V/500mA)(800Ω/0.2Ω) = 19.656k
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:
Time = (3 Hours)(C_TIMER/0.1µF)
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_CC disables the
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 100µA 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
V⁺
7
V_CC
LTC1731
2
CHRG
V_DD
100k
2k
µPROCESSOR
OUT
IN
1731 F01
Figure 1. Microprocessor Interface
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 2k pull-up resis-
tor. Once the charge current drops to 10% of the full-scale
current (C/10), the N-channel MOSFET is turned off and a
100µA current source is connected to the CHRG pin. The
IN pin will then be pulled high by the 2k pull-up. 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.
Refer to Table 1 for the summary.
Table 1. CHRG Pin Interface with Microprocessor
IN
OUT
STATUS
Low
High
Charging
Low
Hi-Z
Charging
High
High
C/10
Low
Hi-Z
C/10
High
Hi-Z
Stop Charging
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 100µA current source to ground.
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