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TPS62177: Buck Regulator Failure with Input Voltage Sag

Craig C
Prodigy 10 points
Part Number: TPS62177

Hi, 

We have a PCBA in a product that uses a TPS62177DQCT buck regulator to regulate 25V to 3.3V. Several of the regulators have failed on products and my team has been testing the PCBA to determine the root cause. 

We've measured the input voltage and never see it exceed the 28V input rating but there is a part of the device operation that can cause a significant voltage sag on the input to the regulator. We are wondering if this rapid sag could damage the regulator and cause it to fail. 

The schematic is below:


Here is an oscilloscope capture of the input voltage transience measured across C37. 



Other things we have tested / checked:
-Confirmed the current usage is well below the 500mA rating (we use max 100mA)
-Checked for voltage transience on the input above 28V during different parts of the device operation
-Confirmed the voltage levels on the enable and sleep signals are below their max rated voltages
-Checked the 3v3 output to confirm nothing is leaking onto it to make it too high
-Check the PCBA operating temperature

Thanks for your help


  • 0
    TI__Mastermind 18235 points

    Hi Craig

    Does this Vin sag happen when power up or after the circuit operate for a while?

    Can there be a load transient on the 3.3Vout side and is there any current limit on the input side?

    Can you please reproduce the Vin sag performance and under what condition does it happens? Can you please also measure the waveform for the SW, Vout (and Iout if possible) ?

    Thanks and best regards.

  • 0 in reply to Gui He
    Prodigy 10 points

    Hi Gui,

    Thanks for your response. 

    -The VIN sag happens after the circuit has been operating for a while
    -There is load transient on the 3v3 out when different ICs operate but not specifically during this input voltage sag
    -No current limit on the input side

    The Vin sag is caused when the Vin power rail connects to an external device with significant capacitance. There is significant current inrush to the other device which causes the Vin power rail voltage to sag. The Vin power rail can supply many amps of current so the 3v3 regulator is not the cause of the sag. 

    -I can not measure Iout due to the PCBA layout

    Here are some scope captures from a sag event:




    Here are closer captures of the SW pin during stable operation:



    Thanks for your help

  • 0 in reply to Craig C
    TI__Mastermind 18970 points

    Hi Craig,

    Bella will give you response by end of this week. Thanks!

    BRs

    Lucia

  • 0 in reply to Craig C
    TI__Mastermind 18235 points

    Hi Craig

    Seems the Vin sag is caused by the sudden connection to an capacitor, which will cause inrush current charging the capacitor from 0 to Vin. This is not caused by the device and need to search solution from the system aspect. 

    Thanks and best regards.

  • 0 in reply to Gui He
    Prodigy 10 points

    Hi Gui, 

    I am aware that the sag is not caused by the TPS62177. My question is, could the TPS62177 be damaged by the sag on Vin?

  • 0 in reply to Craig C
    TI__Mastermind 18235 points

    Hi Craig

    The Vin sag will not cause damage to the TPS62177. T

    Thanks and best regards.