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AMC3330: Decoupling power

Tomasz Jastrzebski
Prodigy 202 points
Part Number: AMC3330

Tool/software:

Hi TI,

Are additional parallel decoupling capacitors really required?
The reason why I ask is because this precisely is what PCB design gurus like Rick Hartley and dr Eric Bogatin call 'legacy code'.
That is; a pattern blindly followed into new ceramic capacitors era without any proof of its effectiveness 'just to be safe'.

So I dare to ask: was it really tested and/or anyhow proven that additional C1, C3, C6 and C8 capacitors are effective and needed?

Kind regards,
Thomas

  • +1
    Guru 279980 points

    The internal DC/DC converter generates large current spikes on both input and output supplies, so large capacitances are needed to prevent ripple and EMI.

    The text in section 7.4 discusses the electrical characteristics of the recommended capacitors, and says that "these listed components have been validated during the development of the AMC3330". The linked application note shows measurements.

  • 0 in reply to Clemens Ladisch
    Prodigy 202 points

    Hi Clemens,

    No slightest doubt large capacitors are needed. The question is if smaller, parallel values are required as well.
    Just because this design has been validated does not mean the circuit would not work equally well without C1, C3, C6 and C8 capacitors.
    I take your answer as a bit evasive so I have to do tests on my own.

  • 0 in reply to Tomasz Jastrzebski
    Guru 279980 points

    Small capacitors are needed to filter out high-frequency current spikes caused by digital switching; this requirement is the same as with any other logic device.

    If the large capacitors are right near the corresponding pins, and if their packages are small enough that the impedance (especially ESL) is small enough, then it is possible that the 1 nF caps do not make much of an improvement. But I would not start a design with this assumption.

  • 0 in reply to Clemens Ladisch
    Prodigy 202 points

    Thank you, Clemens. I make no presumptions. My initial design includes additional capacitors per official app notes but I will try to remove them, test and measure - mainly to save PCB space, which is very limited.