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LM27761 switching spurs: is there a way to reduce amplitude of the switching spurs

RJ Hopper
Mastermind 18095 points
Other Parts Discussed in Thread: LM27761, LM27761EVM

Greetings:

I am using the LM27761 to generate -1.8V from a +5V supply.  Current load is 180 mA.  I am using this as part of a high end communication system.  The 2 MHz switching spurs seem to be particularly high in amplitude.  Using an RF probe in proximity to the top of the device yield a spur at +8 dBm.

I am using the recommended circuit from the D/S with a couple of exceptions.  I changed the input capacitor to a 10 uF (Ceramic) and I changed the output capacitor to a 47 uF (ceramic).

I am finding that the switching spurs are coupling onto other power planes on the board and then getting to the output spectrum.  The other planes are separated by ground planes but they are near or underneath the LM device.  I think the switching spurs are powerful enough to "get through" the shielding of the ground planes and are getting picked up by the other supply planes.  I have tried adding additional bypass capacitors and output filters but it has no impact.  This is why I think that it is a radiated effect.

Are there any tricks to minimize the amplitude of the switching spurs on this device?  Long term, is there a different device that I should consider for next revision?

  • 0
    TI__Intellectual 1120 points

    Hi,

    Did you replace LDO output capacitor or charge pump output capacitor? If you have not replaced charge pump output capacitor, that is one thing you could try. You might have to also increase value of the flying capacitor. Another thing is DC bias and its effect to capacitors. Have you checked how much your capacitors are affected by DC bias? For example, in LM27761EVM, there is 10 uF input capacitor. According to manufacturer, its effective capacitance is reduced to ~2.4uF when input voltage is 5V. 

    How much ripple you see in input, LDO output and in charge pump output with 180 mA load current?

    Regards,

    Juha

  • 0 in reply to Juha Karttunen
    TI__Mastermind 18095 points
    I have tried increasing the capacitance value of input, output, CPout, and flying. No reduction of coupled spurs. The conducted spurs are not a problem as the output cap does indeed help reduce these. My issue is the switching spurs are coupling downward to an internal layer power supply plane. There is a ground layer in between but it looks like the spurs are strong enough to "get through". I am resigned to re-layout the board to isolate the DC-DC converters. Thanks for the suggestions. --RJH