In the LT8645S, when an external supply is used for VBIAS, what is the expected behavior of the device if VIN drops to 0V?

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Accepted Answer

Expected Behavior When VIN = 0V and VBIAS is Supplied Externally: 
 
 1. The LT8645S will not operate: 
 The device requires VIN to be present (minimum 3.0V typical) to power the internal power switches and enable switching. Even if VBIAS is externally supplied, VIN = 0V disables the switching regulator, and no output voltage will be generated. 
 2. VBIAS does not substitute VIN: 
 VBIAS is used to power internal control circuitry to improve efficiency, especially at higher VIN. It cannot power the switching stage or replace VIN as the main input supply. 
 3. No damage occurs if VBIAS is present while VIN = 0V, as long as voltage ratings are respected: 
 The absolute maximum rating for VBIAS is 25V. 
 4. Power sequencing matters: 
 It’s generally recommended to apply VIN before or at the same time as VBIAS. If VBIAS is applied first, and VIN is 0V, the device remains off, but some leakage current may flow, depending on the state of other pins.

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LT8645S: FAQ