Data Sheet

LT8645SA/LT8646SA

analog.com

Rev 0

32 of 39

state. If the EN pin is grounded the SW pin current will drop to near 1µA. However, if the VINpin is grounded while the

output is held high, regardless of EN, parasitic body diodes inside the LT8645SA/LT8646SA can pull current from the

output through the SW pin and the VINpin. Figure 62 shows a connection of the VIN and EN/UV pins that will allow the

LT8645SA/LT8646SA to run only when the input voltage is present and that protects against a shorted or reversed

input.

Figure 62. Reverse VINProtection

Thermal Considerations

For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the

LT8645SA/LT8646SA. 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

LT8645SA/LT8646SA. 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 LT8645SA/LT8646SA 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 LT8645SA/LT8646SA power

dissipation by the thermal resistance from junction to ambient.

The internal overtemperature protection monitors the junction temperature of the LT8645SA/LT8646SA. If the

junction temperature reaches approximately 180°C, the LT8645SA/LT8646SA will stop switching and indicate a fault

condition until the temperature drops about 10°C cooler.

Temperature rise of the LT8645SA/LT8646SA is worst when operating at high load, high VIN, and high switching

frequency. If the case temperature is too high for a given application, then either VIN, switching frequency, or load

current can be decreased to reduce the temperature to an acceptable level. Figure 63 shows examples of how case

temperature rise can be managed by reducing VIN, switching frequency, or load.

Figure 63. Case Temperature Rise

VIN

VIN

D1

LT8645SA/

LT8646SA

EN/UV

GND

DEMO BOARD

VIN = 12V, fSW = 2MHz

VIN = 24V, fSW = 2MHz

VIN = 12V, fSW = 400kHz

VIN = 24V, fSW = 400kHz

VIN = 48V, fSW = 400kHz

LOAD CURRENT (A)

0

1

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8

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100