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TPS25944A: Suspicious behavior, probably reverse current related

Reno
Prodigy 160 points
Part Number: TPS25944A
Other Parts Discussed in Thread: TPS25940-Q1

Hello,

I'm using the TPS25944A as an eFuse according to the attached schematic. The circuit needs quite a lot of capacitors because of other schematic aspects.

What I observe is that the nFLT output never gets high and the output is switched on and off with a frequency of about 100Hz if there is a low resistive load (<100mA). Increasing the load leads to an increased frequency of the nFLT pulses and out ripple. If the load is around 1A everything gets fine and the nFLT is constantly high.

My only guess for this behavior is that the reverse current comparator is triggered by a current flowing from the output capacitors to the input capacitor. I would not have expected this and unfortunately this generates a lot of noise. There is no possibility that current is fed from outside. Current can only come from the capacitors. The capacitors are low ESR types (<20mR).

PGOOD is OK all the time.

Is there a way to prevent this toggling for low loads/currents?
Are there other reasons that might cause this behavior?

I would appreciate any help...

Thanks and regards,
Reno


  • 0
    TI__Guru** 106251 points
    Hi Reno,

    Is it happening in steady state or during startup ?
    Can you send test waveform with Vin, Vout, nFLT and input current.
    Does the DC-DC converters has good local decoupling capacitors?

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 160 points

    Hi Rakesh,

    This is happening in steady state (all the time). The startup is quite slow (~20ms) and looks as expected.

    At my request there are attached the waveforms for Vout and nFLT (in correlation).
    Vin looks more or less the same as Vout as the ripple propagates to the input (the amplitude is a bit lower). If the eFuse is disabled then there is no periodic ripple on Vin. Because of the structure of the PCB it is not so easy to get a waveform for the input current.

    I've noticed that my uploaded schematic was not correct in regards to the DC-DC converters. I've attached an updated version.

    The DC-DC converters have local decoupling at <100uF, not 'really' good. But, there is a second eFuse using the same IC (TPS25944A) that works fine with the same Vin. The difference is that this eFuse has much less output capacitance compared to the faulty one. I also thought about the DC-DC converters or the Vin ripple as a cause for this behavior, but this should also affect the other eFuse - what is not the case.

    Best Regards,
    Reno

  • 0 in reply to Reno
    TI__Guru** 106251 points

    Hi Reno,

    On the second eFuse, there is DC-DC converter load ,So, Vin and Vout are following the same ripple. The ripple from DC-DC converter is appearing at 12V-IN.

    At no-load or light load on first eFuse, the Output voltage remains fixed. The differential voltage is causing reverse comparator to trigger for the First eFuse.

    Can you probe differential voltage across each of the eFuse. ?

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 160 points

    Hi Rakesh,

    Thanks for your thoughts. I'm sure your explanation is correct. 12V-OUT is very stable because of the capacitors, and any ripple on Vin leads to a 'false' triggering of the VERY FAST reverse comparator.

    I've attached two waveforms for Vin where you can see a lot of ripple. It may come from the DC-DC converters or from the source (POE). In any case it is not so easy to substantially reduce this ripple in my design.

    In the end, the ripple on Vout is not so much a problem compared to the 'false' nFLT signal. So I guess, I need to find an eFuse that has not the reverse current comparator or a comparator that is much slower (>>1us).

    Unfortunately, I have no differential probe. But, I think I can imagine what it would show.

    I will try to add some extra capacitors to the DC-DC inputs. This might influence the effect. But, in the end I'm certain I've to change the type of eFuse. The design is a prototype, so it's still possible to alter it.

    Would you agree?

    Thanks and regards,
    Reno


  • 0 in reply to Reno
    TI__Guru** 106251 points
    Hi Reno,

    Please switch to TPS25940-Q1 device, it has higher reverse comparator threshold (6x times higher than TPS25944A). So, TPS25940-Q1 will be less susceptible to these kind of noise and avoids false trip.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 160 points
    Thank you, Rakesh.