Output capacitor in buck converter

Question

I am working with the buck converter AOZ1284 Datasheet

I want to get 5V with 12V in input so I chose R1=52.3k and R2=10k (Vout = 0.8 x (1+R1/R2) =0.8x(1+52,3k/10k)=4.984V). Also I fixed L=4.7uH.

How do I calculate the value of Co and Cin (input and output capacitors?)

I found a section in the datasheet about the output capacitor but I don't know

• the acceptable value for DLETA_Vo
• the cut off frequency for the LC filter in the output

Answer 1

For future use, use Q = C * V (charge = capacitance times voltage).

If you take the derivative with respect to time, you get

• dQ/dT = C * dV/dT + V * dC/dT

If we define dQ/dT = current (I), and define dC/dT = ZERO, we now have

• dQ/dT = I = C * dV/dT

Now let us use that for ripple of 0.01 volt, for 1microSecond delay time, with 1 amp load:

• I = C * dV/dT ===>> C = I * dT/dV

and we substitute

• C = 1 amp * 1uSecond / 0.01 volt = 1uF/0.01 = 100 microfarad

Notice this does not bring up the Resonance of the Inductor with this Capacitor which affects the SwitchReg regulation_loop (negative feedback) stability and phase margin.

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Read the comment by "BobT" for appropriate caution on Capacitor ESR.

Answer 2

You are expected to specify the value of \$\Delta V_O\$ yourself.

How much ripple do you want to allow on your output?

• Running a microprocessor from it? You might allow 50mV of ripple.
• Running an audio amplifier? You might want ripple lower than 1mV.

The \$f\$ referred to in the equations for \$\Delta V_O\$ is the switching frequency of the regulator.

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You might attack it backwards. Take a common capacitor value, then calculate the ripple and ask yourself if it is acceptable. No, pick a bigger capacitor and try again.

Answer 3

The capacitor values depend on whether you are trying to make a physically small converter , or if the input is from a transformer + rectifier, or you want a low cost design.

If the input is already smooth DC , then for Cin you just need a small low ESR capacitor. one or two 22uF 35v ceramic capacitors , or a handful of 10uF 35V ceramic capacitors, X5R would be cheapest option. If you have a transformer / rectifier then you will need some bulk energy storage, like 1000uF electrolytic.

For Cout you will need another couple of 22uF ceramic capacitors, e.g 2 x 22uf or 4 x 10uF if running at 400kHz , half that if running at 800kHz . Running at a higher frequency allows smaller capacitors, but reduces efficiency (i.e. runs hotter). The TI website has some useful tools , including web-bench. Here is a snip from the LMR36520 datasheet , this is commonly used on the Adafruit and pololu DC-DC modules.

As with most power supply electronics, don't try running on the hairy edge, if you need a 1A supply , then use a 2A rated device.

The Pololu 12v-5v 1A module is here https://www.pololu.com/product/2831 (you can see the schematic if you scroll down) it uses 4 x 10uF ceramics on the output, and only one on the input. You can get 10uF ceramics in an 0805 package at a better cost point than 22uF in a 1206, for roughly the same space. Designers and manufacturers try to minimise the variety of parts in their designs, for example I use 10uF/35v in 1206 , 10uF/10v in 0805 and 1uF/50v in 0805. A full reel (5000) of 10uF/35v cost less than a half reel of 22u/35v.