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LM5021: internal 5.2V source is Vin or Vcc

Zion79
Prodigy 50 points
Part Number: LM5021

Hello,

I am wondering whether the internal 5.2V gets power from Vin pin or Vcc pin. To reduce BOM cost, I don't want to put too much capacitance on Vcc pin. Vcc pin is estimated to have 125mVpp ripple. Can this ripple have adverse effect on any internal circuit powered by 5.2V rail? 

Thanks,

Zion

  • 0
    TI__Mastermind 31580 points

    Hi Zion,

    Vcc ripple will propagate through any down stream regulators if the output of the regulators is brought externally to another pin then it will have its own decoupling capacitor on the pin which will reduce the ripple.

    For internal references the amount of capacitance is limited as it must be fabricated in silicon and takes area so that is why it is best to do as much ripple reduction as possible on Vcc before it enters the IC.

    Regards

    Peter

  • 0 in reply to Peter Meaney
    Prodigy 50 points

    Hi Peter,

    The Vcc pin voltage is not used by any external circuit. I am only worrying about whether the internal circuit of LM5021 can be adversely affected by the 125mVpp ripple on Vcc pin. If yes, what is the acceptable Vcc ripple threshold for the internal circuit of LM5021? I can then add capacitance to meet the criteria. 

    Thanks,

    Zion 

  • 0 in reply to Zion79
    TI__Mastermind 31580 points

    Hi Zion,

    The ceramic capacitor fitted to the Vcc is a ceramic of 100nF to 1uF and than should ensure the internal reference rail have low ripple voltage. I would start with 100nF and increase the value if you run into problems during development.

    Please refer to section 9 and 10 for power supply recommendations and layout guidelines as the PCB layout will also contribute to the ripple voltage on the Vcc cap.

    Regards

    Peter