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STM32WB55: Power supply ripple/pulsation causing BLE TX to fail

Aku
Associate III

‎2021-09-20 04:58 AM

We've switched to new DC-DC(LM3281) for main power supply and found that BLE TX stopped working correctly and not passing packet test anymore.

After deeper investigation we found that our DC-DC in ECO mode has ripple/pulsation of 60mV/50uS, which causing this issue.

Switching DC-DC into PWM mode solves the issue, but power consumption raise makes this option undesirable.

Issue can be reproduced on Nucleo board with Transparent VCP test firmware and CubeMonitor-RF in packet test mode.

Questions:

  • How clean power supply should be and where this information can be found?
  • Is there any way to expose radio phases that requires clean power supply to GPIO?
  • Is it bug in core2 firmware or expected behavior?
10 REPLIES 10
Aku
Associate III
In response to Aku

‎2022-03-23 01:57 AM

Hi,

Yes, here is what we got:

0693W00000KdhJOQAZ.png 

Changes from reference design:

  • L replaced with LR series to damp rising front.
  • 2nd stage LC filter

Downsides:

  • Series R introduces small voltage drop under heavy load (not critical for us)
  • Ripple is still here, WB is ok with it, but other ICs may not be that happy
  • Not reference design, you must verify DCDC stability by your self

Also official comment from Ti FAE:

Running LM3281 in FPWM would be the easy and best way to reduce the ripple, if the higher quiescent current is acceptable.
Adding a second LC to the output has the potential to make the loop unstable. If the additional poles from this filter are within the loop bandwidth, the device can become unstable. This may not be evident when operating in PFM mode. I would suggest to run a loop stability measurement in PWM mode by injecting a signal into the FB pin. This can give an idea by how much you need to shift the poles of your 2nd LC to higher frequencies. But keep in mind the effectiveness of the filter to filter the PFM ripple reduces then.
And you can also find more information about designing a second stage LC for the link: https://e2e.ti.com/blogs_/b/powerhouse/posts/design-a-second-stage-filter-for-sensitive-applications

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