Datasheet

14
LT1307/LT1307B
1307fa
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
present on the SW pin and the V
IN
pin as shown in Figure
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
V
IN
SW
LT1307
1N4001
1307 F18
2 OR 3
CELLS
GND
V
IN
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
V
IN
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
APPLICATIO S I FOR ATIO
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