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TPS737: Unexpected oscillation happen in the application

Part Number: TPS737
Other Parts Discussed in Thread: TPS7A91, TPS7A80

Hi,

I am using TPS73701 as 3.3V LDO in the application.

Cin on Vin is 1uF and Cout on Vout is 10uF and 1nF is applied.

When Vin is supplied from power supply(Vout=3.6V, Imax set to 2A, Iout = 200mA) , LDO is oscillating which make multi tone to single tone RF signal.

Have you seen the issue on it?

By putting big capacitor in the output of Power supply, it can be disappeared. But it looks unusual.

I attached the application and measurement.

  • Hi Junghwa,

    If I am reading your plot correctly the oscillation is at 900 MHz for it's fundamental - is that correct?

    If so that is well outside the loop bandwidth crossover (typically 100kHz to 10 MHz for most LDOs), so if that is the case it will not be the LDO causing the oscillation. What is your load?

    Regards,
    David
  • Hi David,

    Yes. 860MHz is the expected signal and you can see the other frequency signals which is modulated about 40KHz.

    When a condenser in the power supply output is added or 22uF capacitor in the Cin of LDO will remove the modulated signal around 860MHz.Lo

    Load current is 200mA which I mentioned as Iout from power supply.

    condenser in the power supply output picture

    22uF in the Cin of TPS73701

    CW capture after adding capacitor 

  • Hi Junghwa,

    Thanks for the additional details and pictures!

    My best bet would be that the noise is coming from the input supply - the PSRR of the TPS737 dips in that area, and you didn't leave too much room for dropout with the 3.6 V min input. If you increase you input voltage to 5V, do you still have the issue?

    You may also want to try the TPS7A91 (not P2P) or the TPS7A80 (P2P) which have better PSRR in this range.

    Regards,
    David