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LM3281: power supply for an MSP432, including ADC and DAC

Thomas Bewley
Expert 1030 points
Part Number: LM3281
Other Parts Discussed in Thread: TPS65279, DAC124S085, TLV9004, , TPS7A05

I have a high-power 6V-18V power source, and will be knocking it down to 5V with a TI TPS65279, which will introduce a ripple at 500kHz (which I will endeavor to filter some, but a significant 500kHz ripple will certainly remain).  From this high-power 5V line, I want to generate a clean low-power 3.3V line, for driving an MSP432P401R, including associated ADC and DAC circuits (using a DAC124S085 and a TLV9004).  Is the LM3281 a good choice for doing this?

If I filter out the 6MHz ripple as suggested by the inductor/capacitor choice in the LM3281 datasheet, can I also assume the the remaining 500kHz ripple (from the 5V line to which this component will be attached) will also be substantially reduced?  Can this be quantified at all?

Thanks in advance for the help!

  • 0
    TI__Expert 6330 points
    Hi Thomas,

    Typically you would prefer to power a processor from a cleaner power source than a switching regulator. Is there a reason you are not using an LDO to step down from 5V to 3.3V?

    Thanks,
    Aaron
  • 0 in reply to Aaron Goodson
    Expert 1030 points

    Corner condition is coming down from 18V.  Seems like I need a switching regulator (like the TPS65279) to first get in the vicinity of 5V without undue heat generation - that will introduce the 500kHz ripple in my 5V line (which yes I will filter some) that I'm chiefly worried about here.  The LM3281 (with high efficieny and a 6MHz switching frequency) is associated with some further filtering, and based on what I've read seems to generate a remarkably clean power source.  If I used an LDO alone, like the TPS7A05, to step from my noisy 5V line to 3.3V, wouldn't the 500kHz ripple just pass right through?  It thus seemed to me like the LM3281 high-freq switching regulator would be well suited for the job of stepping down from my noisy 5V line to a much cleaner 3.3V line.  But yes, using an LDO in such an application is apparently more typical than using a switching regulator.  That is why I am posting the question here...  

  • 0 in reply to Thomas Bewley
    TI__Expert 6330 points

    Thomas,

    LDOs are designed to reject ripple from their input voltage. The spec for this is the power supply rejection ration (PSRR). The LDO that you referred to has a PSRR of 30dB at 500kHz. There is an application note that has an overview of certain specifications of LDOs that can be found here. 

    Regards,

    Aaron

  • 0 in reply to Aaron Goodson
    Expert 1030 points
    Thank you! That is a very useful quantitative measure of what happens at 500kHz using the TPS7A05. Can the PSRR at 500kHz of the LM3281 (with the passives suggested in the datasheet, L_SW=0.47 µH, C_IN=C_OUT=2.2 µF) be similarly estimated? I think very little ripple will be left at 6MHz, my concern is really what passes through at 500kHz...
  • 0 in reply to Thomas Bewley
    TI__Expert 6330 points
    The LM3281 does not spec for PSRR. So it cannot be quantified as easily.
  • 0 in reply to Aaron Goodson
    Expert 1030 points
    Ok, I capitulate. I'll use an LDO, with a high PSRR, for the last stage of the voltage reduction. Thanks.