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LM43603: Output abnormal issue due to touch with sensitive pin

Tess Chen
Intellectual 2675 points
Part Number: LM43603


Hi Team:

Issue: the circuit could run as normal, but once hand touch with some sensitive area on the board, the outout disapear and sometime could revovery when moving the connection and occationally this cause  short between the output to ground. Could you please help review the schematic and part of the PCB to figure out the reason? We assume that the senitive reason is related to FB pin.

Trial: If you touch the pin or peripheral device with the multimeter probe (turn on the voltage or turn off the multimeter), the same phenomenon will appear;

However, there is no problem when using 6pF ~ 12PF oscilloscope probe to test the pin;

Please find the schematic left and right part in the below two pictures:

The screenshot of PCB is as follows: 24 V enters on the right side and 5 V outputs on the left side; the green wiring is FB (surface layer), and the green box in the figure is the sensitive area.

The figure below is another PCB. The schematic diagram is basically the same, but it also has the same problem. The right side is 24 V input, the left side is 5 V output, the green is FB.

  • 0
    TI__Mastermind 28020 points

    Hi Tess,

    My first thought is that the large FB resistors may be causing noise to couple into the FB. 

    If you try reducing the values to 100kΩ and 24.9kΩ, does it improve?

    Regards, Jason

  • 0 in reply to J Arrigo
    TI__Intellectual 2675 points

    Hi Jason,

    Thanks for your feedback. Please find the Vout Spike with FB pin touch through multimeter probe(Power OFF status).

    Question:

    • Does the spike is caused by loop unstable due to additional cap brought by Probe at FB pin ? Or the over voltage or current brought by the probe add?
    • I want to avoid that as the spike may destroyed the load.

    Best Regards,

    Tess Chen

  • 0 in reply to Tess Chen
    TI__Mastermind 28020 points

    Hi Tess,

    Yes, I believe the high impedance on the FB pin is picking up noise when you place your finger or probe on this node .

    Have you tried reducing the FB resistors? 

    Regards, Jason

  • 0 in reply to Tess Chen
    TI__Mastermind 35210 points

    Hello Tess,

    This does not appear as stability issue.

    Not exactly sure if it is due to probing method.

    I recommend monitoring the output.

    Whatever is on the FB pin will be a gained up version on the output.

    FB pin is a sensitive pin.

    This experiment will help answer some of the above questions.