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:
