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TPS650861: 50Hz noise at PMIC BUCK output during short power down and power up interval

Yunjing Wang1
Expert 4300 points
Part Number: TPS650861
Other Parts Discussed in Thread: , MSP430F5529, IPG-UI

Hi team,

My customer is using TPS650861，and they tested 50Hz noise at BUCK output after power up the PMIC.

Both BOOSTXL-TPS650861 EVM and customer own PCB have this issue.  

It will only appear when the power is turned on and off frequently, such as power on again after 10s of power down.  Moreover, it will disappear after several seconds of power-on.

The fault waveforms captured at the moment of power-on are as below, from top to bottom are CTRL, BUCK1, BUCK3, and CTRL triggers BUCK1 to start, and BUCK1 triggers BUCK3 to start.

Can you help to check what might be the reason causing this?  How to verify the possible reasons and how can we solve it?

Thanks and best regards

Yunjing

  • 0
    TI__Genius 17380 points

    Hello,

    If one of the below does not help, please provide the OTP Generator file for further investigation. The schematic may also help.

    The TPS650861 reports faults through the registers, so they can look at those to try to see what may be happening:

    1. SHUTDNSRC - note: the UVLO bit will be set to '1' every time the device powers up so for the sake of this test, it can be ignored

    2. PWR_FAULT_STATUS1

    3. PWR_FAULT_STATUS2

    These can be read by the IPG-UI using the MSP430F5529 Launchpad directly (3.3V only) or through the level translator on the BOOSTXL-TPS650861 board if they need 1.8V.

    If the PMIC detects any fault, it will perform emergency shutdown, reload the OTP, and then check the CTL pins for what to do next. There is an approximately 10 ms mask on power fault during each regulator start-up which could explain why you see the repeating pattern. 

    If you see a CRITTEMP warning, then there may be too much capacitance on BUCK3/4/5. Too much capacitance causes instability which triggers a CRITTEMP. I think this is less likely given that the issue is seen on the BOOSTXL-TPS650861, but the GND pin on that is fairly inductive so it is a slight possibility.

    I would also recommend looking at VSYS, VREF, LDO3P3, LDO5P0, PVIN3/4/5, V5ANA, DRV5V_1_6 to see if any of these are not staying flat during the start-up.