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TMS320F28374S question concerning 2.2uF caps on supply pins

Josh%20Mandich
Intellectual 1155 points

There are 2.2uF caps on the 3.3V analog and 1.2V core supply pin bypass positions on the ‘377S, per the data sheet recommendations.

There are concerns on the analog pins (using 0402, 6.3V, X5R parts from AVX – p/n 04026D225KAT2A), mainly, about the voltage coefficient of these parts, and if the data sheet recommendation accounted for this phenomenon.  The parts will be about ½ their rated value under 3.3V bias.

 

Could you confirm this?  Or what is TI’s recommendation?  Use a bigger, higher voltage part to minimize the capacitance drop on the analog pins?’

  • 0
    TI__Expert 7885 points
    Hello Josh,

    The datasheet does not recommend physical footprint sizes, dielectric types or voltage ratings for the capacitors. That is left up to the discretion of the board designer.

    VDD Signal Description:

    "1.2-V digital logic power pins. If the internal 1.2-V VREG
    is used, place a decoupling capacitor near each VDD pin
    and distribute 12 μF to 26 μF evenly across all VDD pins.
    If an external supply is used, TI recommends a minimum
    total capacitance of 20 μF. The exact value of the
    decoupling capacitance should be determined by your
    system voltage regulation solution."

    If you are not using the internal VREG for the VDD rail, these are purely decoupling caps and the overall total capacitance should meet the requirement of your power IC. If you are using the internal VREG, then the accuracy of these caps is critical and should be comprehended when choosing the voltage and dielectric rating.

    VDDA Signal Description:

    "3.3-V analog power pins. Place a minimum 2.2-μF
    decoupling capacitor to VSSA on each pin."

    The VDDA caps are purely decoupling caps and the accuracy of these caps are not terribly critical.

    Best Regards,
    Adam Dunhoft