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TPS61378-Q1: FB short to GND related question

Chuqiang Wen
Expert 5815 points
Part Number: TPS61378-Q1

Hi Expert,

My customer is using TPS61378-Q1 in their project. We found a wired phenomenon when short FB pin to GND and remove short. Please see the description as below. 

You can find the schematic and test waveform as below. 

Procedure: 

Vin=3.3V, Vout=12V, open load, Fsw=660KHZ

Short FB pin to GND, then Vout keep rising, will trigger the OV protection when rise to 16.5V. 

Then, remove the short on FB pin, VIN will rise from 3.3V to 5.5V as below waveform shows. 

If change the Rfreq from 62K to 20K(Fsw change from 660KHZ to 2MHZ), VIN won't be pulled higher than 3.3V

Question: 

Do you know why VIN will rise higher than 3.3V? Is there any reverse current occur? Some other loads are supplied by 3.3V power rail. So customer don't want to have such phenomenon. 

--------------------------------------------------------------

Fsw=660KHZ Waveform

change Rfreq to 20K waveform

Schematic: 

Thanks!

Ethan Wen

  • 0
    TI__Expert 5180 points

    Hi Ethan,

    Thanks for reaching out. I will check this problem on TI bench and update my analysis early next week. 

    May I ask why does customer have this FB short to GND test? Is this one standard test or this phenomenon will happen in customer's real application?

    And can customer accept change FSW from 660kHz to 2.2MHz? Thanks.

    Best Regards,

    Eric Yue

  • 0 in reply to Eric Yue
    TI__Expert 5815 points

    Hi Eric,

    Thanks! Please let me know if have any update. 

    Please see below feedback from customer. 

    Eric Yue said:
    May I ask why does customer have this FB short to GND test? Is this one standard test or this phenomenon will happen in customer's real application?

    It may happen in real application.

    Eric Yue said:
    And can customer accept change FSW from 660kHz to 2.2MHz? Thanks.

    They think it can't be accepted. Because many component may change accordingly. 

    Ethan Wen

  • 0 in reply to Chuqiang Wen
    TI__Expert 5180 points

    Hi Ethan,

    thanks for your update. I will update my test result in these two days. thanks for the patience.

    Best Regards,

    Eric Yue

  • 0
    TI__Genius 12245 points

    Hi Ethan,

    These two waveforms are what we test in our bench with the parameters provided in customer's schematic. However, customer's problems don't appear in out test, Vin does not rise to 5.5V when recovery from FB short (fs = 660kHz).

    Could you please ask your customer for EN waveform in this case? Because we notice that Vout falls to 5V when recovery from FB short in customer's waveforms, we guess the chip may be disabled and start up again in this process. 

    And more, if this problem occur again, maybe larger input capacitance or inductance could be helpful. Thanks.

    Fsw=660kHz Waveform

    Best Regards,

    Nathan Ding