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Output sine wave on an LDO using the TPS65721

Other Parts Discussed in Thread: TPS65721

We're getting an output sine wave on the LDO of the TPS65721.  It's not consistently present - sometimes when it's under battery power or powered by a wall charger, it's not present.  We've tried reducing the impedance of the resistor dividers to stabilize the output, but that did not work.  

The ripple on the LDO line is 5-10mV, but it's continuous and the amplifiers on the board are amplifying it instead of rejecting it.  I can mess around with the components on the DC/DC converter, but it seems that perhaps changing the LDO components might get better results (perhaps the 22pF feedback capacitor).  

Anyone else have any issues like these?  Thanks very much!  

  • Hi,

    What is the frequency of the oscillation? This frequency should point towards the source of the problem.

    Is there anything else running on the board at the time of the ringing? If there is a large current draw and if layout is problematic the GND can move causing reference to move. This in turn would make the LDO behave oddly and possible ring.

    Please review the layout guidelines and EVM.

    What happens if you load the LDO with a 1 mA load and again a 100mA?