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Original question:

TPS62148: Design Needs More Phase Margin...

TPS62148: Seeing Voltage Ripple on the Input Pin - What's Happening?

Darren (FAE)
Expert 5483 points
Part Number: TPS62148

[Captured Waveform]

As you can see in the attached waveforms, there is ~150mV of "drifting" observed on the input.

Q1: What could be causing this "drifting" at Vin?
Q2: Will such a high "drifting" at the input cause any trouble with the control capability?

  • 0
    TI__Mastermind 21755 points

    Hi Darren,

    Can they confirm how they measured the VIN? Would recommend across the input cap using pigtail technique and only measure the ripple without other measurements to confirm the ripple is low enough. What is the VIN being derived from? I do not see any issues with the TPS62148 being able to suppress the input ripple and regulate as expected. But, they can always add more input cap and additional filtering if they are concerned.

    Thanks,

    Amod

  • 0 in reply to Amod Vaze
    TI__Expert 5483 points

    Hi Amod,

    1) I confirmed the customer is using a Passive probe, and the pigtail technique.

    2) To clarify what they were seeing:
    They tried measuring while adjusting the output load, and saw:
    Increasing the load caused the SW frequency to decrease;
    Then, lowering the bulk capacitance at Vin helped SW frequency increase back to "normal" value

    Since there was this ripple on VIN, they were wondering if it affects the control loop somehow.
    But the TPS62148 is not basing how it controls SW frequency on VIN, right?

    3) VIN is powered from a switching regulator, that switches at 350kHz, so they don't believe this VIN "ripple" is coming from that.

  • 0 in reply to Darren (FAE)
    TI__Mastermind 21755 points

    Hi Darren,

    This looks like a 3.3V design and if the max VIN is below 6V, I would recommend checking the LV portfolio as well. The VIN waveform itself looks quite clean and ripple is ~150mV? They can reduce this further by adding more input cap if required.

    How much switching frequency variation are they observing? Can you share exact details on "lowering bulk capacitance" - what was it before and after? Is MODE pin tied to high (3.3V)? The IC should run in PWM mode only if that is the case. Looking at figure 71 on page 30 of datasheet with FSEL=LOW and MODE=HI or PWM mode, the frequency should actually increase with increasing loads and should remain fairly steady around 1.2-1.4MHz.Can you confirm if the VIN at the IC pins is actually 3.3V or something lower? I would expect the frequency behavior to be identical to the datasheet figure. Are they using their own board or testing on EVM? Possible to share their BOM? 

    Thanks,

    Amod