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LT1239

Backup Battery Management Circuit

Linear Technology 

LT1239

PIN FUNCTIONS

ADJ (Pin 1): Adjust Pin of Regulator 1. The regulator will

servo the adjust pin to 3.75V referred to ground. Bias

current will be approximately 50nA and will flow into the

adjust pin.

GND (Pin 2): Ground Pin for Regulator 1. Note that the

three ground pins (pins 2, 4, 5) are connected together

internally and should all be grounded externally.

SHDN1 (Pin 3): Shutdown Pin for Regulator 1. Regulator

1 output will be on if the shutdown pin is either: 1) Left

floating (open circuit) or 2) pulled up to the 5V rail. If the

shutdown function is not used, the shutdown pin is nor-

mally left open circuit. Regulator 1 output will be off if the

shutdown pin is pulled to ground. The shutdown pin

current with the pin pulled to ground will be in the range of

2µA flowing out of the pin. The shutdown pin current with

the pin pulled up to 5V will be zero.

GND (Pin 4): Ground. This ground pin is tied to the

substrate of the die, between regulator 1 and the rest of the

circuit. It is used as an isolation barrier between regulator

1 and the rest of the circuitry.

GND (Pin 5): Ground Pin for Regulator 2.

SHDN2 (Pin 6): Shutdown Pin for Regulator 2. Regulator

2 output will be on if the shutdown pin is either: 1) Left

floating (open circuit) or 2) pulled up to the 5V rail. If the

shutdown function is not used, the shutdown pin is nor-

mally left open circuit. Regulator 2 output will be off if the

shutdown pin is pulled to ground. The shutdown pin

current with the pin pulled to ground will be in the range of

2µA flowing out of the pin. The shutdown pin current with

the pin pulled up to 5V will be zero.

E/A (IN) (Pin 7): Noninverting Input of the Error Amplifier.

This pin should be tied to the center tap point in the output

divider for regulator 1. The bias current for this pin will be

in the range of 3nA and it will flow out of the pin.

E/A (OUT) (Pin 8): Output of the Error Amplifier. This is

normally connected to the center tap of the backup cells.

NC (Pin 9): Not Connected.

INPUT 2 (Pin 10): Input Pin (VCC) for Regulator 2, the Error

Amplifier, and the Low-Battery Detection Circuit.

IMON 2 (Pin 11): Current Monitor Pin for Regulator 2. If the

current monitor function is not used, this pin should be

tied to the output pin of regulator 2.

OUT 2 (Pin 12): Output of Regulator 2. It is also the

inverting input and output of the comparator. If the main

5V system supply is up and running then the comparator

output will pull the output of regulator 2 up to 5V.

5VIN (Pin 13): Noninverting Input of the comparator and

the collector of the output driver. The collector of the

output driver is normally connected to the main 5V system

supply.

INPUT 1 (Pin 14): Input Pin (VCC) of Regulator 1.

IMON 1 (Pin 15): Current Monitor Pin for Regulator 1. The

current flowing out of this pin will be approximately 1/200

of the output current of regulator 1. If the current monitor

function is not used, this pin should be tied to the output

pin of regulator 1.

OUT 1 (Pin 16): Output of Regulator 1.

FUNCTIONAL DESCRIPTIO

Regulator 1: Regulator 1 is used to supply the charging

current to the backup batteries. It converts the voltage on

the main battery to a fixed output voltage to charge the

backup cells. The output voltage is set with a voltage

divider connected between the output and ground with a

tap point of the divider connected to the adjust pin. The

regulator servos its output in order to maintain the adjust

pin at 3.75V referred to ground. The resistor divider

should be chosen such that the divider current is approxi-

mately 5µA. This means the impedance from the adjust pin

to ground should be approximately 750kΩ. For safety

requirements a resistor can be placed between the output

pin and the top of the divider that sets the regulated output

voltage. The regulator will regulate the voltage at the top of

the divider. Quiescent current will be 10µA to 15µA. Output

short-circuit current will be approximately 70mA.

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