LM5121: Cbst question

Ben Kuo

Part Number: LM5121

Tool/software:

Dear team,

Customer use LM5121 for different watt application, but face to boot strop capacitor question during their design.

In LM5121 EVM, we use PSMN4R0-40YS MOSFET and the total gate charge(Qg(tot)) is up to 31nC.

Base on datasheet equation 33, Cbst need to be 206.7nF~253nF for this HS FET. But in EVM, We place 0.1uF which is invalid from datasheet.

My question is:

Does Qg in datasheet stand for Qg_total or Qgs only?

Regards,

Ben

+1 Stefan Schauer 25 days ago

HI Ben,

thank you for supporting the LM5121.

You have to use the total gate charge for this calculation.

As the description says:

"ΔVBST is the tolerable voltage droop on CBST, which is typically less than 5% of VCC or conservatively 0.15 V"

In this case it was not the conservative 0.15V use but the less 5% of VCC. (VCC = 8.3V which means less the 3.45V).

Best regards,

Stefan

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