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TPS54356: Getting a Vout that is 2.8V instead of 3.3V

Luc Charbonneau
Prodigy 50 points
Part Number: TPS54356

Hey,

We setup our circuit based on the datasheet and the output of the TPS54356PWPR is undervolted. Supposed to get 3.3V but getting 2.8V.

I tried to adjust the switching frequency to 695KHz, 500KHz, and 400KHz but I haven't seen any change in the output. Right now I have a 113k resistor which should make the switching frequency 400KHz

We put the LSG in our design with the mosfet: BUK7M6R3-40EX.

There are a couple of loads attached to the circuit mostly ICs with a few resistors which act as pull-ups for a few GPIOs, but they're all in parallel so it shouldn't affect Vout.

This is a snapshot of the circuit:

I'm looking for ideas on how to troubleshoot this issue.

  • 0
    TI__Expert 7320 points

    Hi Luc,

    The schematic looks good.

    Is it possible the input voltage is below 4V and lead to an output voltage drop?

    Also if the load current is too high, the output voltage may be below 3.3V.

    Please show me some waveforms such as input voltage, output voltage, PH voltage and inductor current for further analysis if possible.

    BTW, have you seen this issue on several boards or just this one? 

  • 0 in reply to Athos Zhao
    Prodigy 50 points

    The input voltage of the chip is a solid 5.2V, which is powered by a USB-C block that can output 3A at 5V. It doesn't go through any FETs or resistors or anything, its a direct supply.

    I did notice something interesting though. This whole thing is part of a computer we're building that is a System-on-Module. This module is powered by the same 5V. When the SoM is connected the output of the buck is 2.8V, when the SoM is not inserted into the board, and I trigger the enable pin of the buck manually I get the proper 3.3V. The SoM does not use the 3.3V buck in any capacity whatsoever, it's meant to power other chips on the board that operate in the micro-amps. If the SoM is taking up so much power form the USB-C block I would expect a voltage drop at Vin but I don't, it's a solid 5.2V with no noticeable ripple.

    This is the waveform of Vin with the SoM removed and it outputs 3.3V as expected.

    This is the wave form of the buck powering on and outputting 2.8V with the SoM plugged in.

    Sorry, what do you mean by PH voltage?

    I'm not sure how I could test the inductor current because it's embedded into the circuit board and I don't have any traces I can put an amp meter in series with the inductor specifically.

    At this point I will check to see if there's some sort of circuit issue with the SoM plugged in that could be causing this, but I'm also wondering if there's some way I could modify the buck to adapt to whatever the issue might be; I know the issue is still up in the air at the moment.

  • 0 in reply to Luc Charbonneau
    TI__Expert 7320 points

    Hi Luc,

    Considering the output voltage influenced by SoM, it may be caused by EMI and layout.

    If the VSENSE trace of TPS54356 is laid near a high frequency signal trace/area, it may be coupled and get a bad regulation.

    PH voltage means the voltage of PH pin of TPS54356. It's switching node of this buck converter and the voltage should be PWM waveform.

    It's ok you don't measure the inductor current. However, I still want to check the input and PH voltage for further check.

  • 0 in reply to Athos Zhao
    TI__Expert 7320 points

    Also you can send the layout file here or athos-zhao@ti.com to me for review.

  • 0 in reply to Athos Zhao
    Prodigy 50 points

    This is the waveform of PH pin with the SoM inserted.

    This is the waveform of the PH pin without the SoM inserted.

  • 0 in reply to Luc Charbonneau
    TI__Expert 7320 points

    Hi Luc,

    The PH shows this 2.8Vout is the performance of this converter since the duty cycle is less than the one under 3.3Vout.

    Considering this performance, we can confirm this issue is not related to the input voltage drop since the duty is changing.

    We can also confirm the issue is not related to the output current since this device would run into hiccup mode when Overcurrent Protection is triggered.

    The only possible reason I can come up now is EMI.

    I'll check the layout and show you feedback later.