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bypass capacitor on VCORE pin on MSP430F5437A

lech lachowicz
Prodigy 70 points

Other Parts Discussed in Thread: MSP430F5437A

Is there a range of capacitances that can be used for the bypass on the VCORE pin?  The PLL is used to generate a 1 MHz MCLK from a 32KHz crystal.  What is an acceptable bypass capacitance at that operating frequency? 

  • Guru 227245 points

    The capacitance is listed in the datasheet (or should). In all cases I looked at it it was 470nF. Preferrably a ceramic capacitor (low ESR). And it is no "bypass" capacitance, it is "storage" capacitance for the voltage generator and required for regulator stability. (there are subtle differences)

    The storage capacitance on DVCC must be at least 10 times larger than the one on VCore and is independent of any low-ESR (ceramic) bypass capacitors.

    There is no capacitance change for different speeds. Different speeds require different VCore voltages. The capacitance is unaffected by this, as the regulator doesn't change its timing and current parameters.

  • in reply to Jens-Michael Gross
    Prodigy 70 points

    This is the specification for the capacitor from the MSP430F5437A from the TI website.

    C   VCORE    Recommended capacitor at VCORE      470 nF

    Not 470pf

    Since a minimum or maximum value is not specified there must be an allowable range for the capacitance and may be based on operating frequency.

     

  • in reply to lech lachowicz
    Guru 227245 points

    lech lachowicz said:
    Not 470pf

    Right. Typo (and a bad one indeed). Sorry.

    lech lachowicz said:
    Since a minimum or maximum value is not specified there must be an allowable range for the capacitance

    Not necessarily. If there were no restriction on the value in a given range, why not giving the range? And no recommended value.
    But I agree, the attribute 'recommended' implies that other values are possible too. But then, a 470nF ceramic capacitor usually already has a +-10% or worse tolerance (or is really large and expensive). So my best guess is that 470nF +- the usual tolerances are a good value. A smaller or larger nominal value might result in an effective value that is too low/high.

    lech lachowicz said:
    may be based on operating frequency

    Well, minimum capacitance is based on the voltage regulator operating frequency, which is independent of MCLK. (Would be bad if the regulator would switch off if you stop MCLK). It is, however, possibly controlled by the PMMCOREV setting. Or just auto-adapting to the required voltage and required current. Who knows?
    A too-large capacitance, however, will likely make the regulator less efficient as it has to work harder to fill it (slower rise time) and because of probably highe leakage etc. So the higher the capacitance is, the more it affects the low-power performance. The lower it is, the higher the rist that the regulation becomes unstable and VCORE oscillates.

    Looking at the parts list of our distributor, the only affordable ceramic capacitors in this range are 220nF (which is most likely way too low), 470nF and 1µF (which is most likely much too high). And the 220nF isn't significantly smaller or cheaper than the 470nF. So why bother?

  • in reply to Jens-Michael Gross
    Prodigy 50 points

    Sorry Jens-Michael, a slight typo. I think you meant to say 470nF.

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