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TPS60403 output ripple voltage

Olli Männistö
Prodigy 150 points
Other Parts Discussed in Thread: TPS60403, TPS54040, TLE2072, TPS62120

I need -5V for opamps that need stay in operating range all the way to true 0V. So far so good, they're shouldn't see much load as they're used to bias a feedback circuit. Therefore the TPS60403 seems a good candidate. I also designed TPS54040 VSS circuit but decided it doesn't make sense to have SMPS running on skipping mode.. 

Back end of envelope scribblings indicate that with the 250kHz switching frequency and using the recommended 1uF Co, there would be about 60mVpp ripple with 30mA load. I'd like to go lower than that as the opamp (TLE2072) does not havethat great negative supply rejection ratio on those frequencies. 

Is there any problem using 1206 X5R 22uF capacitor for output? At -5V it'd work out to 14uF real capacitance that'd reduce the ripple to <5mV. I'd keep the freewheeling cap and input cap at 1uF. 

Appnotes recommend using RC filter or LDO. Previous increases output impedance and the latter increases cost but would eliminate voltage droop.

  • 0
    TI__Guru*** 142860 points

    That much capacitance should be ok.  Just go ahead and order the EVM and try it out.

    Another option to look at is the TPS62120/2 in an inverting configuration.  There's an app note on the product page and it will give a regulated -5V while supporting the low ripple you need.

  • 0 in reply to Chris Glaser
    Prodigy 150 points

    I always thought these milliamp-class SMPS circuits were a bit odd. But lets give this one a look, perhaps it'd be a good compromise between a charge pump and a full-flexed SMPS aimed for twenty times the current density. Interestingly the MLCC capacitors take more room than the inductor or SMPS chip.