# Input capacitance estimation for a buck converter

To estimate minimum input capacitance required for a buck converter to help with power stability & noise suppression, I came across below equation by TI.

The input capacitance value depends on duty ratio, switching frequency, output current & maximum input pk-pk ripple voltage.

Can someone please help to understand what drives the spec for allowed maximum input pk-pk ripple voltage? For e.g., if it's specified as 2% of nominal input voltage value then what are the reasons for setting it to 2%, why not 20%?

• I don't think this is the way to go, without considering the drive impedance to the converter input. Assuming you have a 0-ohm output impedance source supplying the buck converter, you theoretically don't need a capacitor. If you now install a front-end EMI filter, assuming all the high-frequency pulses go through the front-end cap and only dc flows from the source, then you would select the capacitor type based on the rms current it can sustain. Commented Apr 14 at 12:15

There are a few things that can drive the spec for input voltage ripple.

One is other things that are attached to the input supply. For example if you have op-amps in a low-noise circuit that are also attached to the input rail you would want to keep the ripple to an acceptable level based on the PSRR of the op-amps and the SNR requirement.

Another is the output ripple of the converter itself. Excessive input ripple can also affect output ripple. It could even affect the operation of the converter itself, maybe by hitting UVLO during load transients for example.

Finally, there's EMI. High ripple on the input can propagate and cause conducted or even radiated EMI which can interfere with other circuitry.

There's a lot of reasons why you would to keep ripple down, namely your output load current. Remember that you have to maintain a steady current so if your output voltage suddenly spikes upwards 20%, your output current would suddenly spikes downwards, due to conversation of energy, potentially making your load current fall to zero. If this happens, you can throw your buck converter into the discontinuous mode region.

John talks about EMI implications as well, so I won't repeat what was said there.

Can someone please help to understand what drives the spec for allowed maximum input pk-pk ripple voltage?

It depends entirely on the switching regulator's data sheet and, what other sensitive circuits are connected on the input supply to the regulator (that might be degraded by the presence of a high ripple).

if it's specified as 2% of nominal input voltage value then what are the reasons for setting it to 2%, why not 20%?

The data sheet for the switching device should indicate the reasons. There are no generic reasons; it's device dependent. But you still need to consider the other sensitive circuits connected on the same rail as the switching regulator's input.