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preventing switcher noise from being injected back on a shared power rail

BrianBrianBrian
Expert 1085 points
Other Parts Discussed in Thread: TPS62173, TPS54120, TPS62170

Hi

I'm designing a mixed signal circuit board that shares its power rail with other circuit boards (third party designs). I would like to use a DC/DC switcher in my circuit to regulate the common +12V rail down to +3.3V for a microcontroller. I've been considering the TPS62173 (on the low end) and the TPS54120 (on the high end). However, I'm worried about noise being injected back on to the common +12V rail and hence affecting other circuits that are using it.

Other than using large input capacitors on the Vin of the DC/DC switcher, what else can I do to minimize the noise injected back on to the power rails? Is there a parameter in the datasheets of TI's DC/DC converters that is relevant here? Of course, an important consideration is the nature of the power supply that is providing the common +12V rail but let's assume that is also out of my control (for example, it could be a linear or switching supply).

Thank you!

  • 0
    TI__Guru*** 142910 points

    I can't recall a customer for which this has been an issue, but we always recommend ordering the EVM (for the TPS62170) and testing your system with it.  Then, copy its schematic and layout into your design and you will get the same performance as you saw with the EVM.

    There is no spec for this as it is entirely application dependent. Some customers use an input inductor to create an input filter.  There are textbooks on how to do this.

  • 0 in reply to Chris Glaser
    Expert 1085 points

    Chris, thank you for the pointer to the EVM.

    However, please let me add further details because I think I didn't word my question very well. The main problem is that the switcher will be running off a common +12V rail that is used by other circuit boards that are outside of my design control (3rd party circuit boards). It's a modular system for audio applications with each  module sharing the power rails. There is no guarantee that other modules have included the power filtering that you speak of and thus could be vulnerable to the switcher in my circuit dumping noise on the power rails. So that is what I would like to avoid if possible.

  • 0 in reply to BrianBrianBrian
    TI__Guru*** 142910 points

    Correct and I don't recall a case where this was an issue.  Multiple power supplies run off a common power bus in numerous systems.  The input filter is not required but would be the choice to mitigate any issue that does arise.

  • 0 in reply to Chris Glaser
    Expert 1085 points

    Actually these other circuits that are running of the common power bus many times do not have their own power supplies/regulators. They just use the common power bus directly to power analog circuitry which is not great design-wise but that is what I'm stuck with.

  • 0 in reply to BrianBrianBrian
    Expert 1085 points

    Also, I've ordered a few EVMs to try some in situ testing but any other nuggets of knowledge that are relevant here would be greatly appreciated!