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TPS542941 output voltage ustable

Key Zhu
Prodigy 20 points
Other Parts Discussed in Thread: TPS542941

I was using a TPS542941 in a power module, with +5v input and outputs a 3.3v and a 1.2v DC power.

The unit works fine both loaded with resistor load and without load. However after soldering to customer's mother board, some of the units experienced output voltage failure, for instance, the output voltage of the 3.3v output raised to 4.77v without any load.

When removed from mother board and tested on a test jig, the unit works fine both loaded and unloaded.

My customer uses my power module to power some FPGAs, and they had a 220uF capacitor directly placed to my output on their mother board.

All the component values and layout are closely kept to the recommended schematic of the TPS542941 datasheet, and please see the attached schematic and PCB layout. Please help me check if there is any problem.  

Now my customer requires a failure report, and I have problem to determine the actual cause.  Please help!  Thanks in advance. pcb.rar

  • 0
    TI__Guru**** 191445 points
    I don't see any particular issue with your schematic or layout. While 220 uF is above the recommended output capacitance, I have seen quite a few designs where 100 - 200uF have been added without difficulty. More likely I think is there may be a leakage path so that some portion of your 5 V supply is appearing at your output.
  • 0 in reply to JohnTucker
    Prodigy 20 points

    Thank you John,

    My question remains, How did the 5V supply leak to the output, there are no direct path, and the input and output pins are GND shielded? 

  • 0 in reply to Key Zhu
    TI__Guru**** 191445 points

    Sometimes there are leakage paths internal to FPGAs or other circuits.  I suggest you probe the circuit to determine if the TPS542941 is trying to regulate to a higher than desired voltage or is being pulled up to that voltage by an external pre-bias voltage.  Check the SW waveforms and measure the duty cycle.

  • 0 in reply to JohnTucker
    TI__Intellectual 2590 points
    Hi Key,

    would you please connect with out Local FAE and sale for the detail support? we need more wavefrom after you connect the 220uF cap and output loading. this problely infulences the stablilty of our IC. or you can test some output ripple, LX, wavefrom and send them to us for more detial debug.
    thanks!

    Eric