8

LTC4008

4008fa

TEST CIRCUIT

–

+

–

+

–

+

EA

LT1055

LTC4008

BAT

7

13

12

CSP

11

0.6V

4008 TC

9

BATMON

90.325k

9.675k

OVERVIEW

The LTC4008 is a synchronous current mode PWM step

down (buck) switcher battery charger controller. The

charge current is programmed by the combination of a

pins. The final float voltage is programmed with an exter-

nal resistor divider and the internal 1.19V reference volt-

age. Charging begins when the potential at the DCIN pin

rises above the voltage at BAT (and the UVLO voltage) and

the ACP/SHDN pin is high. An external thermistor network

is sampled at regular intervals. If the thermistor value

exceeds design limits, charging is suspended and the

FAULT pin is set low. If the thermistor value returns to an

acceptable value, charging resumes and the FAULT pin is

sampling interval for the thermistor.

OPERATIO

As the battery approaches the final float voltage, the

charge current will begin to decrease. When the current

drops to 10% of the full-scale charge current, an internal

C/10 comparator will indicate this condition by latching

the FLAG pin low. If this condition is caused by an input

current limit condition, described below, then the FLAG

indicator will be inhibited. When the input voltage is not

present, the charger goes into a sleep mode, dropping

battery current drain to 15

µA. This greatly reduces the

current drain on the battery and increases the standby

time. The charger can be inhibited at any time by forcing

the ACP/SHDN pin to a low voltage. Forcing ACP/SHDN

low, or removing the voltage from DCIN, will also clear the

FLAG pin if it is low.

Table 1. Truth Table For Indicator States

MODE

DCIN

ACP/SHDN

FLAG**

FAULT**

Shutdown by low adapter voltage (Disabled on LTC4008-1)

<BAT

LOW

HIGH

HIGH

LOW

Normal charging

>BAT

HIGH

HIGH

HIGH*

HIGH*

Input current limited charging

>BAT

HIGH

HIGH*

HIGH*

LOW

Charger shut down due to thermistor out of range

>BAT

HIGH

X

LOW

HIGH

Shut down by ACP/SHDN pin (USER)

X

Forced LOW

HIGH

HIGH

LOW

Shut down by undervoltage lockout

>BAT + <UVL

HIGH

HIGH

HIGH*

LOW

*Most probable condition, **Open-drain output, HIGH = Open with pull-up, X = Don’t care