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TPS7A4001 / Vout drop at 800MHz noise immunity test

Other Parts Discussed in Thread: TPS7A4001

Hi All,

Our customer has evaluated TPS7A4001 and they are performing 800MHz noise immunity test at their PCB. Then they confirmed that Vout is dropped linearly when noise source (antenna) is located closely. I'd like to have your comment for following questions to find workaround.

  • Condition
    • Vin = 55V
    • Vout = 3.3V
    • Iout = less than 10mA
  • Question
    • Do you know such output voltage drop linearly due to RF noise?
    • Is the strength of noise immunity depend on devices? For example is there possibility that this behavior is solved by replacing other device with the same PCB layout?
    • I guess that the FB line is affected due to bad PCB layout. Then is adding capacitor between FB pin and GND effect to noise immunity?

Your comment will be appreciated.

Best Regards,

Sonoki / Japan Disty

  • Hi Sonoki,

    Could you please clarify what you mean by voltage drop linearly? Do you mean that a negative noise spike will result in the output dropping by the same amount?

    At such a high frequency, TPS7A4001 does not offer a PSRR advantage. Any ripple on the input will pass through the device to the output. Once you are in the MHz range of frequencies, PSRR for most LDOs is negligible with the primary source of PSRR coming from the output capacitor. With 55V to 3.3V you have ample headroom. Is it possible to place a filter before TPS7A4001 to filter out the higher frequency noise?

    Very Respectfully,
    Ryan
  • Hi Ryan-san,

    I'm requesting customer to provide waveform but I have not be able to get it.

    In their system, the input source is Li-Ion battery, so I think the input voltage may be stable under RF noise. So I suspected the affect from FB line. Can you give your comment for affect to FB due to RF noise?

    Best Regards,
    Sonoki
  • Hi Sonoki-san,

    At this high frequency, the noise is well beyond the bandwidth of this LDO. A 10pF capacitor from FB to GND may help reduce the affect of the noise if it is on FB.

    Very Respectfully,
    Ryan
  • Hi Ryan,

    In past days, I talked about abnormal behavior of TPS7A4001 at noise immunity test, and then we guess that FB pin may be influenced. Now our customer ask me about following question for FB pin. Can you confirm them?

    1. Is the FB input configured with bipolar process or CMOS process?
    2. Does FB pin have ESD protection diode for both to IN pin and GND pin?

    Your comment will be appreciated.

    Best Regards,

    Sonoki / Japan Disty

  • Hi Sonoki-san,

    Please allow me a day or two in order to check on this information.

    Very Respectfully,
    Ryan
  • Hi Ryan,

    Thank you for your comment. I'm waiting for your update.

    Best Regards,
    Sonoki
  • Hi Sonoki-san,

    The main pass element is CMOS; however, there are also some bipolars on the FB rail.

    There is an ESD diode between FB and GND; however, there is not an ESD diode between FB and IN.

    Very Respectfully,
    Ryan