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All,

Lately I've been thinking more about bulk capacitance in my designs. Previously, I've been careful about part selection and placement for decoupling and filter capacitors during design. Once a board is fabricated, I double check voltage sag on each power rail and make adjustments from there.

I'm not really happy with this approach. I'm not a big fan of crossing my fingers and making adjustments on the fly. Are there any rules of thumb out there or good references on calculating the amount of cap that's needed in a design? As an aside, I work on mostly digital designs, so there are no inductive loads to worry about.

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  • \$\begingroup\$ You need to tell more what the bulk capacitance is for. Is it for 50Hz rectified AC and linear regulator, or for high frequency switching regulator use? \$\endgroup\$
    – Justme
    Dec 13, 2018 at 17:56
  • \$\begingroup\$ You need to know typical and peak current draw from your supply to have any idea about what your capacitance values should be. \$\endgroup\$
    – Reinderien
    Dec 13, 2018 at 18:29
  • \$\begingroup\$ For this project, I'm using a 7.5VDC wall adapter fed into a pair of LDOs to step down to 5V and 3V3. The data sheet for the LDO calls for 10uF on the input side, which seemed a little skimpy. I'm expecting peak current draw to be around 900mA between both LDOs (800mA on 5V, 100mA on 3V3). \$\endgroup\$ Dec 13, 2018 at 19:04
  • \$\begingroup\$ OK, but again you need to tell more or it's just hand-waving. Which LDO type? How much current is available from wall adapter? Is the load plain DC or will there be sudden jumps in the current consumption? What is the load, like is it a bunch of chips like a microcontroller consuming gulps of current at the clock speed? \$\endgroup\$
    – Justme
    Dec 13, 2018 at 19:25
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    \$\begingroup\$ What are the specs for the 7.5 VDC wall adpter that feeds the LDOs? Is it linear regulated, linear unregulated, or switch mode power supply? Have you measured the unloaded voltage it gives out? For the regulated cases, there is no real need for any large bulk capacitance on the LDO input as there is no mains ripple, just what the datasheet says is necessary for low input impedance. For unregulated, it depends on how much there is bulk in the supply. For the bulk capacitance on LDO output, the datasheet must be checked for min/max capacitance and min/max ESR for the output to be stable. \$\endgroup\$
    – Justme
    Dec 14, 2018 at 18:25

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