LT8640 datasheet(21/26 Pages) AD

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Shorted and Reversed Input Protection

The LT8642-1 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 LT8642-1 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 LT8642-1 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 LT8642-1’s output. If the VIN

pin is allowed to float and the EN pin is held high (either by

LT8642-1

Rev. 0

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APPLICATIONS INFORMATION

a logic signal or because it is tied to VIN), then the LT8642-

1’s internal circuitry will pull its quiescent current through

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

current draw 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 LT8642-1

can pull current from the output through the SW pin and

the VIN pin. Figure 7 shows a connection of the VIN and

EN/UV pins that will allow the LT8642-1 to run only when

the input voltage is present and that protects against a

shorted or reversed input.

Figure 6. Paralleling Two LT8642-1s

LT8642-1

86421 F06

CLKOUT

LT8642-1

SYNC/MODE

COUT

VOUT

20A

Figure 7. Reverse VIN Protection

VIN

LT8642-1

EN/UV

86421 F07

GND

Thermal Considerations

For higher ambient temperatures, care should be taken in

the layout of the PCB to ensure good heat sinking of the

LT8642-1. The ground pins on the bottom of the package

should be soldered to a ground plane. This ground should

be tied to large copper layers below with thermal vias;

these layers will spread heat dissipated by the LT8642-

1. Placing additional vias can reduce thermal resistance

further. The maximum load current should be derated

as the ambient temperature approaches the maximum

junction rating. Power dissipation within the LT8642-1

can be estimated by calculating the total power loss from

an efficiency measurement and subtracting the inductor

loss. The die temperature is calculated by multiplying the

LT8642-1 power dissipation by the thermal resistance

from junction to ambient.

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