I have a step down DC/DC regulator. I want to regulate VIN:12V and Vout:1.2V @2A
What type of package do I need to choose for my bypass capacitor?
If I choose 0402 capacitor instead of 0805, will the operation of the regulator be affected?
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\$\begingroup\$ What does the DC-DC datasheet say about capacitor value? \$\endgroup\$– bobfluxCommented Dec 5, 2021 at 10:38
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\$\begingroup\$ Sorry, I forgot to write that the capasitor value is 22uF. \$\endgroup\$– user295040Commented Dec 5, 2021 at 10:42
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1\$\begingroup\$ If you can find 22uF in a 0402 size, then you will not like the price. I doubt anyone makes 22uF in that small of a package. I doubt you can even get one in 0805. \$\endgroup\$– JRECommented Dec 5, 2021 at 11:10
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\$\begingroup\$ Then you have the dc bias derating to consider. \$\endgroup\$– KartmanCommented Dec 5, 2021 at 11:11
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\$\begingroup\$ And the ripple current rating of a 0402... \$\endgroup\$– bobfluxCommented Dec 5, 2021 at 18:02
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1 Answer
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What type of package do I need to choose for my bypass capacitor?
If you are considering 22 uF in 0402 then you need to research the data sheet for the specific size from the specific manufacturer (if they are even made in that size). You may have a hard time extracting that data from the manufacturer, but persist, it's there, and it's generally not in their short-form catalogues.
The reason for this is that high-K ceramic capacitors have a vicious voltage coefficient of capacitance, and the higher the K, that is the more capacitance in a smaller package, generally the higher the voltage coefficient.
I don't know why capacitor manufacturers are allowed to specify the capacitance at 0 V bias, and their working voltage, without also having to specify the capacitance at the working voltage. At working voltage, it can be 30% of what they publish. When you factor in an initial tolerance, and a temperature ceofficient as well, you may only get 4 to 5 uF from a capacitor you bought as 22 uF, at its working voltage.
Solutions - run the capacitor at less than its working voltage, provide much more nominal capacitance than the regulator needs for stability, and get those detailed data sheets.