LTC1730-4/LTC1730-4.2
8
sn1730 1730fs
Stop Charging
The charger is off when any of the following conditions
exist. The voltage at the V
CC
pin is less than 40mV above
V
BAT
(sleep mode), or the potential at the NTC/SHDN pin
is less than 50mV (shutdown mode). The trickle charge
current source and the internal pass transistor are turned
off. The internal resistor divider is disconnected to reduce
the current drain on the battery when in sleep mode.
Input Voltage (Wall Adapter)
The input voltage to the LTC1730 must have some
method of current limit capability. The current limit level
of the input power source must be lower than the
overcurrent limit (I
MAX
) set by the sense resistor (I
MAX
=
100mV/R
SENSE
). If a wall adapter without current limit is
used, or the current limit level is above I
MAX
, the charger
will turn on briefly and then immediately turn off after the
overcurrent condition is detected. This cycle will be re-
sumed every 400ms (C
TIMER
= 0.1µF) until the total charge
time has run out. If overcurrent protection is not needed,
short the SENSE pin to V
CC
.
Trickle Charge and Defective Battery Detection
At the begining of the charge cycle, if the cell voltage is low
(less than 2.45V) the charger goes into a 35mA trickle
charge mode. If the low cell voltage persists for one
quarter of the total charge time, the battery is considered
defective and the charge cycle is terminated. The CHRG
pin output is then forced to a high impedance state.
Battery Charge Current
The battery charge current is determined by the current
limit of the input supply (wall adapter). However, this
current must not exceed the maximum charge overcurrent,
I
MAX
. If an overcurrent condition is detected, the charging
is immediately terminated, the GATE pin is pulled to
ground and the charge pump turns off. The charging will
resume after a 400ms time off (C
TIMER
= 0.1µF).
Programming the Timer
The programmable timer is used to terminate the charge
and sets the minimum ON/OFF time and the overcurrent
time-off period. The length of the timer is programmed by
an external capacitor from the TIMER pin to ground. The
total charge time is:
Time (Hours)= (3 Hour)(C
TIMER
/0.1µF)
The timer starts when the input voltage (at least 40mV
greater than V
BAT
) is applied and the potential at the NTC/
SHDN pin is between 0.5 • V
CC
and 0.875 • V
CC
. After a
time-out has occurred, the charging stops and the CHRG
pin becomes high impedance.
CHRG Status Output Pin
This open-drain output requires a pull-up resistor and can
be used to indicate three charging conditions. When fast
charging begins, an N-FET (capable of driving an LED)
turns on, pulling this pin to ground. Once the duty cycle at
the GATE pin drops below 10%, the N-FET turns off and a
40µA current source to ground turns on. When a time-out
occurs or the input supply is removed, the CHRG pin goes
high impedance indicating that the charge cycle has ended.
By using two different value resistors, a microprocessor
can detect three states from this pin: charging, C/10 and
stop charging (see Figure 1).
APPLICATIO S I FOR ATIO
WUU
U
CHRG
1730 F01
V
CC
LTC1730
V
+
V
DD
2k
620k
3
2
OUT
IN
MICROPROCESSOR
Figure 1. Interfacing with Microprocessor
When the LTC1730 is in charge mode, the CHRG pin is
pulled to ground by an internal N-MOSFET. To detect this
mode, force the digital output pin, OUT, high and measure
the voltage at the CHRG pin. The N-MOSFET will pull the
pin low even with a 2k pull-up resistor. Once the charge
current drops below 10% of the full scale current (C/10),
the N-MOSFET is turned off and a 40µA current source is
connected to the CHRG pin. By forcing the OUT pin to a
high impedance state, the current source will pull the pin
low through the 620k resistor. When the internal timer has
expired, the CHRG pin becomes high impedance and the
620k resistor will pull the pin high to indicate that charging
has stopped.
