LTC1731-8.2/LTC1731-8.4
APPLICATIONS INFORMATION
When the LTC1731 is in the charge mode, the CHRG pin
is pulled to ground 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
resistor. 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.
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 and the 100k resistor
will then pull the pin high to indicate the charge cycle has
ended.
End of Charge (C/10)
The LTC1731-8.2/LTC1731-8.4 include a comparator to
monitor the charge current to detect a near end-of-charge
condition. This comparator does not terminate the charge
cycle, but provides and output signal to indicate a near full
charge condition. The timer is used to terminate the
charge cycle. 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 100µA current
source to ground. After an internal time delay of 320ms,
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 LTC1731-8.2/LTC1731-8.4 drive an external
P-channel MOSFET to supply current to the battery. The
DRV pin is internally clamped to 6.5V below V
CC
. This
feature allows 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
charging current error. The gain of the current amplifier is
around 0.6µA/mV. For every 1µA of base current, a 1.6mV
gain error shows up at the inputs of CA. With R
PROG
=
19.6k and 100mV across R
SENSE
, this gain error causes a
1.67% error in charge current.
Constant-Current Only Mode
The LTC1731-8.2/LTC1731-8.4 can be used as a pro-
grammable current source by forcing the TIMER pin to
V
CC
. This is particularly useful for charging NiMH or NiCd
batteries. In the constant-current only mode, the timer and
voltage amplifier are both disabled. An external termina-
tion method is required to properly terminate the charge.
Stability
The charger is stable without any compensation when a
P-channel MOSFET is used as the pass transistor.
However, a 10µF capacitor is recommended at the BAT
pin to keep the ripple voltage low when the battery is
disconnected. A ceramic output capacitor may also be
used, but because of the very low ESR and high Q
characteristics of multilayer ceramic capacitors, it may
be necessary to add a 1Ω resistor in series with the
ceramic capacitor to improve voltage mode stability.
When a PNP transistor is chosen as the pass transistor, a
1000pF capacitor is required from the DRV pin to V
CC
. This
capacitor is needed to help stablize the voltage loop. A
10µF capacitor at the BAT pin is also recommended when
a battery is not present.
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LITEON [ LITE-ON TECHNOLOGY CORPORATION ]
