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Pre-Biased DC-DCs: any problem with that?

Denis Alain1
Intellectual 600 points
Other Parts Discussed in Thread: TPS54395

Hi,

I'm looking at the TPS54395 for a new design. This DC-DC has the "Non-Sinking Pre-Biased Soft Start" feature, which sounds a great feature, but I never used this kind of feature.

My question is simple: Is there any problem using a DC-DC with pre-biased output feature when I don't need it? Why this feature is not present on all DC-DCs?

Thanks,

Denis

  • 0
    TI__Guru**** 191445 points

    Typically a synchronous buck converter will discharge any pre-bias voltage back thru the low side FET at start up.  If the low side switch stays on long enough, very high sinking currents can result.  When the low side switches off the first time, the inductor current must remain continuous and the current will be forced back to the input thru the high side FET body diode during the dead time and then thru the switch itself when the high side is turned on.  This may result in over voltage conditions on the input (depending on how much energy was available in the pre-bias condition and the resulting sinking current).  Other users just don't want the non-monotonic start up of the pre-bias voltage discharging.  Various ICs use different schemes to circumvent these issue and are generally marketed with phrases like you see in the TPS54395.  Generally they are completely safe to use in non pre-bias applications.

  • 0 in reply to JohnTucker
    Intellectual 600 points

    Thanks for the answer! It sounds clear to me!

    Regards, 

    Denis