LM27222: Permanently leave High Side FET on

Gerald

Part Number: LM27222

Hi Experts,

May we ask for help? Our customer would just like to confirm if it is possible to leave the high side FET on for an indefinite amount of time or is there a time-limitation? Do he need to switch the low side FET on before activating the high side FET?

We will wait for your confirmation. Thank you so much.

Kind regards,
Gerald

over 3 years ago

+1 Alexander Mazany over 3 years ago

TI__Genius 11770 points

Hello Gerald,

Using the bootstrap technique to generate the high-side bias means that the high-side FET cannot be continuously on. The low-side FET must be switched enough that the bootstrap capacitor has some time to recharge. See this document for a good explanation. Whether or not the low-side switches first or not is minor, the reason for this is that a high-side input that has no bias yet will simply skip, and then the low-side will charge on the next cycle until everything is properly biased.

There is a way to have 100% duty cycle with some extra components. The first way is to replace the bootstrap circuitry with an isolated power supply referenced to HS. Another way is to implement a charge pump as shown in this document. Both of these methods have extra cost associated with them, but can be used for generating a bias for a continuously on, high-side FET.

thanks,

Alex Mazany

0 Gerald over 3 years ago in reply to Alexander Mazany

TI__Genius 9580 points

Hi Alex,

Thank you so much for your detailed response. We will let you know if we have further concern.

Kind regards,
Gerald

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LM27222: Permanently leave High Side FET on