LTC4002
11
4002f
APPLICATIO S I FOR ATIO
WUUU
Undervoltage Lockout (UVLO)
An undervoltage lockout circuit monitors the input voltage
and keeps the charger off until V
CC
rises above the UVLO
threshold (4.2V for the 4.2 version, 7.5V for the 8.4
version) and at least 250mV above the battery voltage. To
prevent oscillation around the threshold voltage, the UVLO
circuit has 200mV per cell of built-in hysteresis. When
specifying minimum input voltage requirements, the volt-
age drop across the input blocking diode must be added
to the minimum V
CC
supply voltage specification.
Trickle Charge and Defective Battery Detection
At the beginning of a charge cycle, if the battery voltage is
below the trickle charge threshold, the charger goes into
trickle charge mode with the charge current reduced to
10% of the full-scale current. If the low-battery voltage
persists for 30 minutes, the battery is considered defec-
tive, the charge cycle is terminated and the CHRG pin is
forced high impedance.
Shutdown
The LTC4002 can be shut down by pulling the COMP pin
to ground which pulls the GATE pin high turning off the
external P-channel MOSFET. When the COMP pin is re-
leased, the internal timer is reset and a new charge cycle
starts. In shutdown, the output of the CHRG pin is high
impedance and the quiescent current remains at 3mA.
Removing the input power supply will put the charger
into sleep mode. If the voltage at the V
CC
pin drops below
(V
BAT
+ 250mV) or below the UVLO level, the LTC4002
goes into a low current (I
CC
= 10µA) sleep mode, reducing
the battery drain current.
CHRG Status Output Pin
When a charge cycle starts, the CHRG pin is pulled to
ground by an internal N-channel MOSFET which is capable
of driving an LED. When the charge current drops below
the End-of-Charge threshold for more than 120µs, the
N-channel MOSFET turns off and a weak 25µA current
source to ground is connected to the CHRG pin. This weak
25µA pull-down remains until the timer ends the charge
cycle, or the charger is in manual shutdown or sleep mode.
After a time out occurs (charge cycle ends), the pin will
become high impedance. By using two different value re-
sistors, a microprocessor can detect three states from this
pin (charging, end-of-charge and charging stopped) see
Figure 1.
V
CC
2k
390k
V
DD
OUTCHRG
µPROCESSORLTC4002
IN
4002 F02
Figure 1. Microprocessor Interface
OPERATIO
U
mode, dropping I
CC
to 10µA. This will greatly reduce the
current drain on the battery and increase the standby time.
A 10kΩ NTC (negative temperature coefficient) thermistor
can be connected from the NTC pin to ground for battery
temperature qualification. The charge cycle is suspended
when the temperature is outside of the 0°C to 50°C
window (with DALE NTHS-1206N02).
To detect the charge 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 resistor. Once the charge current drops below the
End-of-Charge threshold, the N-channel MOSFET is turned
off and a 25µA current source is connected to the CHRG
pin. The IN pin will then be pulled high by the 2k resistor
connected to OUT. Now force the OUT pin into a high
