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UCD9246: Using a switcher to provide 3.3V

SteeVeeDee
Intellectual 700 points
Part Number: UCD9246
Other Parts Discussed in Thread: LMZM23601, LP5907

Is there any concern about using a buck converter to supply V33A, V33D, and V33DIO of the UCD9246?  I have a LMZM23601 providing 3.3V on the board, and for simplicity and efficiency I would like to use it to power the UCD9246. 

Will there be any problems with this or do I need to provide some sort of "quiet" voltage to V33A? 

We don't have room for the FCX491 shunt regulator in the data sheet.

Thanks, Best, Steve

  • 0
    TI__Mastermind 24075 points
    Hi Steve,

    Our engineer who is familar with this part is not available now.
    I try my best to answer your question.

    The waveform on LMZM23601 datasheet shows the output ripple is small, so I think it should be fine use LMZM23601 to power V33A, V33D and V33DIO.

    Thanks
    Qian
  • 0 in reply to Qian Chen
    TI__Genius 13700 points

    I have seen applications that incorporate a 1 ohm resistor between the V33D and V33A to knock down any noise that may be present.  Typically, they have put a 4.7uF capacitor on each rail along with the resistor.  Switcher output would flow to V33D then pass through 1 ohm to the V33A rail with the decoupling on both rails.

  • 0 in reply to Brad
    Intellectual 700 points

    Perhaps I'll throw down an external LDO anyway.  I have a 5.3V rail as well, and the LP5907 is very small compared to an external pass transistor.

  • 0 in reply to SteeVeeDee
    Intellectual 700 points
    The LP3996SD-3333 is a dual LDO with a PG output- I could run individual LDO's to V33A and V33D and use the PG output to release the UCD9246. What do you think of this idea? I'm a little paranoid about digital contamination of the V33A domain.
  • 0 in reply to SteeVeeDee
    TI__Genius 13700 points
    Any issues I've seen with the UCD have typically been insufficient capacitance and a customer who incorporated a ferrite bead between the 3V3A and 3V3D into their EVMs. That customer's customers then replicated their design possibly not using the same bead which caused an oscillation on the analog rail. That's when I started recommending the low value resistor as an alternative. But we have had quite a few who have tied them directly together without issue and none that I know of that have had a specific issue with doing that.
    We have also had a number of customer's power the UCD with a small switching controller without issue.

    I've never seen or tried powering each rail with a unique LDO. I do not know if that would be a good idea to attempt as there could be issues with timing between the rails (not specifically aware of any but just worries me a bit).
  • 0 in reply to Brad
    Intellectual 700 points
    I put a single LDO powering both 3V3A, 3V3D- the LDO is powered from a switcher to reduce some of the losses. Thanks., Best, Steve