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TPS54061: Unknown Source of Noise Frequency on SW Node

Part Number: TPS54061
Other Parts Discussed in Thread: LM5161

Hello, I am using TPS54061 with the following parameters:

VIN=10-40V, nominal 24V

VOUT = 1.8V

Iout = 20mA max, 5mA nominal

Initially my circuit was designed for full 200mA, but in reality actual load current was more like 20mA max. Thus, my original circuit was pulse skipping quite a lot. I only noticed this because I have some sensitive signal conditioning on the same board that was picking up the noise in ADC data (frequency band 10-40kHz). I verified the source was TPS54061 by taking a long scope capture on switch node and doing FFT.. the shape of spectrum on FFT matched the ADC noise spectrum exactly.

Then, I tried loading down the output with 50ohm load, so additional 36mA or so, which seemed to mostly move out of pulse skipping region, but noise on ADC was still present. 

Moving to TPS54061EVM, I modified the circuit as follows, to try and get full CCM operation:

  • L1 -> 660uH (2x CBC2518T331M in series)
  • R4 -> 6.2kOhm
  • C5 -> 0.1uF
  • C6-> 150pF
  • R6 -> 13.3kOhm

With 360ohm load (~5mA), I still get pulse skipping and the bad spectrum. Increasing the loading to 90ohm (~20mA) gets the converter out of pulse skipping, but the spectrum is still bad (spikes roughly every 2.5kHz). 

I also tried adding another 330uH inductor in series to get 990uH total, but this made no appreciable difference, aside from making the noise spikes slightly closer in frequency (maybe every 2.4kHz instead of 2.5kHz). 

Can someone from TI please comment on the source of the noise on switch node FFT and any way to remove it with this converter?

Schematic:

Spectrum (20mA load):

Switch Node:

  • Hi Evan,

    Even with the additional 36mA of load current your device may still be pulse skipping which could account for why you're still getting noise on your ADC. I notice you're switching at 400kHz, you may consider decreasing your maximum switching frequency from 400kHz.

    The datasheet gives equations 6 and 7 to help select an appropriate switching frequency, but another quick check would be to use the following equation:

    fsw_max=(1/ton_min)*(VOUT/Vin_max)

    Ton_min for this device is typically 120ns, using this and your design parameters I get a max switching frequency of 375kHz.

    Alternatively, your design could be unstable or there could be something else on the board that is causing the noise.

    With such a light load, you'll want a device with FPWM so that the device remains in CCM even at light loads. This way you'll avoid that pulse skipping. You may consider a device such as LMR36520FADDAR which offers FPWM.

    Another alternative device is LM5161 which also offers FPWM.

    Does this help answer your question?

    Regards,

    Harrison Overturf