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UCC27201A: HO and LO difference due to Vf?

Ella KIM
Expert 4212 points
Part Number: UCC27201A


Hi Team,

I observe LO and HO level difference - LO 10V and HO 9V. I thought it's because of the forward-voltage of internal boot diode, but it looks there is no voltage drop in datasheet (figure 26 and 27). Could you advise why there's a voltage difference on LO and HO? Can any other factor cause this difference?

Here's the schematic. FYI, D61 is DNI (Do Not Insert).

Thank for your advice.

  • +1
    TI__Genius 11770 points

    Hello Ella,

    The voltage difference is because of the boot diode. The datasheet screenshot with the blue circle shows exactly why there is a difference. Just to recap, LO is pulled to VDD, while HO is pulled to the bootstrap voltage which is about VDD - Vf. If you want to check, you can measure the bootstrap voltage across the bootstrap capacitors and see if it is VDD - Vf like expected. The reason you see no drop in the figure 26/27 is because of the way that test is setup. Usually you cannot have HO and LO in phase like that because it wouldn't allow the bootstrap to charge. I'm not sure exactly how the test was set up, but I would guess that the HS node was grounded and VDD was applied to the HB pin. Hopefully this was helpful.

    thanks,

    Alex M.

  • 0 in reply to Alexander Mazany
    TI__Expert 4212 points

    Hi Alex,

    It helps. Thank you for your clear answer.

    Could I ask one additional question? During the normal operation, I should consider V_FI, not V_F. Is it correct?

    Then under what operation is V_F considered?

  • +1 in reply to Ella KIM
    TI__Genius 11770 points

    Hello Ella,

    It depends on a few factors, it is mostly a matter of the components selected rather than the operation. Startup tends to have a higher current draw as the bootstrap capacitor charges up. Once everything is charged, the current draw will be smaller in steady-state. This guide is for external bootstraps but the information should be helpful. You have a fairly large bootstrap capacitor and no resistor, so you will likely be closer to the high-current case. If you already have a board, you may be able to just measure the current in the bootstrap, or figure out a rough estimate of the drop by calculating VDD - HO. 

    thanks,

    Alex M.