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I was reading about the selection of output capacitors at the output of buck converters and found this Link

The accepted answer states that before I select an output capacitor, we must fix the desired ripple current and then choose the inductor based on the ripple current.

  1. How to choose and fix the ripple current? Should we just assume the ripple current to be 20 to 40% of Iout max current as mentioned in the TI App Note and proceed with those calculation steps.

I am not sure I get this. I always thought that the selection of output capacitor was based on ripple voltage.

For example, If I have an IC Whose Vcc ranges from 0.8V to 1.2V and typical Vcc is 1V. My Buck will produce 1V with a ripple that must not exceed the Min and Max Vcc of the IC. So, my ripple should be less than 200mV. So, here, obviously our ripple voltage is very important. If the ripple voltage is very high, then our 1V IC may get damaged. But we also calculate ripple current.

  1. How is the ripple current more important that the ripple voltage. What happens if we have high ripple current and low ripple current?

So, I think we would select a capacitor based on our calculated ripple voltage (To be less than 200mV).

  1. So, is our estimate of Inductor ripple current same as the output capacitor ripple current ?

I tried other app notes also. But I couldn't find a formula that mentions the calculation of output capacitor ripple current. And can someone tell me how output ripple voltage and output capacitor ripple current are related and which is calculated first and based on what parameter.

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  • \$\begingroup\$ I believe the main reason to select ripple current is to ensure the converter stays in continuous mode at the lowest output currents and to stay out of saturation at the highest currents. It also affects \$I^2R\$ heating. It isnt as much about output quality like it is with voltage. \$\endgroup\$ – DKNguyen Nov 5 '19 at 3:04
  • \$\begingroup\$ So, is our inductor ripple current the same as the output capacitor ripple current? Could you add some answer to my questions in points? An example would help as I am trying to understand which to calculate first \$\endgroup\$ – Newbie Nov 5 '19 at 3:07
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    \$\begingroup\$ Your capacitor ripple current rating must be at least as high as your ripple current so it doesnt cook. You make inductor and cap ripple as equal for calculation to use the smallest cap and go with a larger cap if reduced voltage ripple is needed. So I guess technically you choose it for inductor first since inductor sizing determines the ripple current. Then you choose a suitable cap that can handle it. \$\endgroup\$ – DKNguyen Nov 5 '19 at 3:11
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    \$\begingroup\$ "So, is our inductor ripple current the same as the output capacitor ripple current?" - Yes. \$\endgroup\$ – Bruce Abbott Nov 5 '19 at 3:36
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Sure the output ripple volts can not be too high in your application .Ripple current heats up the output caps due to their ESR .The life of electrolytic caps decreases with temperature .You know the ripple current ,so with the ESR of the caps you can get power dissipation and heating .If you have concerns about product life you can use better caps or place more in parallel to get temperatures down .

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  • \$\begingroup\$ Why is the ripple current important? Is the ripple current factor only important for the Capacitor or does it affect the IC connected to the downside? \$\endgroup\$ – Newbie Nov 5 '19 at 4:47

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