9

LTC1731-4.1/LTC1731-4.2

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 dis-

abled in trickle charge mode.

Gate Drive

Typically the LTC1731 controls an external P-channel

MOSFET to supply current to the battery. The DRV pin is

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

charge current error. The gain of the current amplifier is

around 0.6

µA/mV. For every 1µA of base current, a 1.6mV

error in charging current.

Constant-Current Only Mode

The LTC1731 can be used as a programmable current

useful for charging NiMH or NiCd batteries. In the con-

stant-current only mode, the timer and voltage amplifier

are both disabled. An external termination 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 10

µF capacitor is recommended at the BAT

pin to keep the ripple voltage low when the battery is

disconnected.

When a PNP transistor is chosen as the pass transistor, a

capacitor is needed to help stablize the voltage loop. A

10

µF capacitor at the BAT pin is also recommended when

a battery is not present.

SENSE

DRV

4.7k

1k

2.2k

1k

AMBER

1.5k

20k

0.2

Ω

19.6k

Li-ION

CELL

LTC1731-4.2

BAT

CHRG

8

7

10

µF

1

µF

1731 TA04

0.1

µF

Si9430DY

6

1

5

4

+

2

3

TIMER

PROG

GND

GREEN

MMBT3906

MMSD4148

MBRM120T3

OPTIONAL: FOR REVERSE

INPUT PROTECTION

MMBT3906

CONDITION

NO WALL ADAPTER

CHARGING (I > C/10)

CHARGING (I < C/10)

TIMER EXPIRED

GREEN

OFF

OFF

ON

ON

AMBER

OFF

ON

OFF

OFF

Using CHRG Pin to Indicate Charge Status

TYPICAL APPLICATIO