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Part Number: LM5122-Q1
Can you tell me the soft-start time when the Vin>=Vout and the 100% duty? Will the output voltage suddenly ramp up? Or does SS pin current charge Css with 10-uA soft-start current source?
If Vin>=Vout, Vin will pass through to Vout. If the Vin is already applied, but the IC is disabled, then the highside FET body diode will pass the input power to the output, andVout=Vin-Vf, where Vf is the body diode of the highside FET. Once the IC is enabled, the 10uA SS current source will charge Css to reach 1.2V threshold, and the high side FET is turned on, and you will see a Vout sudden jump to from (Vin-Vf) to Vin.
If you enable the IC at the moment you apply Vin, you may not see the Vout voltage jump because the FET may already be turned on before the output voltage is established.
Youhao Xi, Applications Engineering
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