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LT1239

Backup Battery Management Circuit

Linear Technology 

LT1239

APPLICATIONS INFORMATION

Using the Current Monitor Function

The current monitor pin outputs a current proportional to

the output current of the regulator. Both regulator 1 and

regulator 2 have independent current monitor pins. The

current monitor function can be used to monitor charge in

the backup cells, to set up a constant current output or to

adjust the current limit of the regulator. The current

monitor pin should be tied to the output pin if the current

monitor function is not used. This will minimize quiescent

current.

The current output of the current monitor pin can be

converted to a voltage by feeding the current monitor pin

output current through a resistor. The voltage across the

resistor will be proportional to output current. This signal

can be used to monitor the output current for either

regulator. Regulator 1 output current is equal to the charge

current for the backup batteries plus the load current of

regulator 2. If regulator 1 output current is greater than

regulator 2 output current, the difference between the

currents is the charge current for the backup cells. If

regulator 2 output current is greater than regulator 1

output current, the difference between the currents is the

discharge current for the backup cells. By integrating the

difference between regulator 1 output current and regula-

tor 2 output current the total charge in the backup cells can

be determined.

Constant Current Charging

NiCd backup batteries are normally charged with a con-

stant current trickle charge. This can be accomplished

7-24V

+

IN 1

OUT 1

LT1239 IMON1

SHDN1 GND ADJ

ICHARGE

+

10µF

(3.75V)

NiCd

BACKUP

BATTERY

R1

+

2µF

VSHDN1

< 0.25V

> 2.8V

NC

ICHARGE

OFF

ON

ON

R1

=

3.75V

ICHARGE

×

220

Figure 3. Constant Current Charging

LT1239 • F03

using regulator 1 and the circuit shown in Figure 3.

In this circuit the voltage at the adjust pin is proportional

to the output current. Regulator 1 will servo its output to

force 3.75V at the adjust pin. The output current will be

scaled from the current monitor pin current by a ratio of

220:1. Output current is equal to 220 × current monitor pin

current. The output current is set by choosing resistor R1,

in the formula shown in Figure 3. Regulator 1 will source

a constant current as long as the voltage at its input is

greater than the battery voltage plus the dropout voltage of

regulator 1. External power monitoring circuitry can be

used to shutdown regulator 1 to terminate charge when a

low current sleep mode is desired.

Setting Current Limit Using the Current Monitor Pin

With the addition of some simple external circuitry the

current monitor pin can be used to control the output

short-circuit current of the regulator. As shown in Figure

4, the current monitor pin can be tied to ground through

a resistor to generate a voltage proportional to output

current. When the voltage across R3 is equal to approxi-

mately 0.6V (one VBE) Q1 will turn on and pull down on the

shutdown pin of the regulator. Q1 effectively steals drive

current from the regulator to limit the output current. C1

is needed to roll off the gain of Q1. Current limit can be set

using the formula shown in Figure 4. This circuit can be

used with either regulator. The shutdown function can

also be used. An open-collector gate connected in parallel

with Q1 can shut down the regulator.

MAIN

BATTERIES

IN 1

+

OUT 1

LT1239 ADJ

SHDN2 GND IMON1

R2 + 10µF

NiCd

R1

BACKUP

BATTERY

R3 =

0.6V

ILIM

×

220

Q1

2N3904

+ C1

R3

2µF

LT1239 • F04

Figure 4. Reducing Current Limit

Using the Comparator

The comparator in the LT1239 is intended to be used as an

automatic switchover circuit between the main 5V

9

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