LT8613 datasheet(23/30 Pages) AD

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LT8645S-2

Rev. A

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APPLICATIONS INFORMATION

Shorted and Reversed Input Protection

The LT8645S-2 will tolerate a shorted output. Several

features are used for protection during output short-cir-

cuit and brownout conditions. The first is the switching

frequency will be folded back while the output is lower

than the set point to maintain inductor current control.

Second, the bottom switch current is monitored such that

if inductor current is beyond safe levels switching of the

top switch will be delayed until such time as the inductor

current falls to safe levels.

Frequency foldback behavior depends on the state of the

SYNC pin: If the SYNC pin is low the switching frequency

will slow while the output voltage is lower than the pro-

grammed level. If the SYNC pin is connected to a clock

source, floated, or tied high, the LT8645S-2 will stay at the

programmed frequency without foldback and only slow

switching if the inductor current exceeds safe levels.

There is another situation to consider in systems where the

output will be held high when the input to the LT8645S-2

is absent. This may occur in battery charging applications

or in battery-backup systems where a battery or some

other supply is diode ORed with the LT8645S-2’s output.

If the VIN pin is allowed to float and the EN pin is held high

(either by a logic signal or because it is tied to VIN), then the

LT8645S-2’s internal circuitry will pull its quiescent current

through its SW pin. This is acceptable if the system can

tolerate several μA in this state. If the EN pin is grounded

the SW pin current will drop to near 1µA. However, if the

VIN pin is grounded while the output is held high, regard-

less of EN, parasitic body diodes inside the LT8645S-2

can pull current from the output through the SW pin and

the VIN pin. Figure 4 shows a connection of the VIN and

EN/UV pins that will allow the LT8645S-2 to run only

when the input voltage is present and that protects against

a shorted or reversed input.

Figure 4. Reverse VIN Protection

VIN

LT8645S-2

EN/UV

8645S-2 F06

GND

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