14

LT1307/LT1307B

REVERSE BATTERY CONSIDERATIONS

The LT1307 is built on a junction-isolated bipolar process.

The p-type substrate is connected to the GND pin of the

LT1307. Substrate diodes, normally reverse-biased, are

16. When the battery polarity is reversed, these diodes

conduct, as illustrated in Figure 17. With a single AA or

AAA cell, several hundred milliamperes flow in the circuit.

The LT1307 can withstand this current without damage. In

laboratory tests, the LT1307 performed without degrada-

tion after sustaining polarity reversal for the life of a single

AA alkaline cell.

When using a 2- or 3-cell supply, an external protection

diode is recommended as shown in Figure 18. When the

battery polarity is reversed, the 1N4001 conducts, limiting

reverse voltage across the LT1307 to a single diode drop.

This arrangement will quickly deplete the cells’ energy, but

it does prevent the LT1307 from excessive power dissipa-

tion and potential damage.

Figure 18. 1N4001 Diode Protects LT1307 from Excessive Power

Dissipation When a 2- or 3-Cell Battery is Used

SW

LT1307

1N4001

1307 F18

2 OR 3

CELLS

GND

LT1307

1307 F16

GND

1 CELL

1.5V

D2

SW

D1

Q1

Figure 16. LT1307 Showing Internal Substrate Diodes D1 and D2.

In Normal Operation Diodes are Reverse-Biased

LT1307

CURRENT

FLOW

1307 F17

GND

1 CELL

– 1.5V

D2

SW

D1

Q1

Figure 17. When Cell Is Reversed Current Flows through

D1 and D2