For a boost converter circuit using a MT3608 IC (Datasheet) I need an input and output capacitor. The datasheet suggests 22μF for each and X5R. I also know the input (3-4.2V) and output (5V) voltage, which also narrows down the search for an approriate capacitor (at least 10V).
However, I am having difficulties finding what footprint that capacitor should have.
All of my compenents are 0603 so it would be neat if the capacitor could have that size too. But I have seen people putting giant capacitors on their boost converters. Some forum threads I have found suggest to choose them "as big as possible in volume" which does not help me at all.
Is there anyone that can give me a rule of thumb for which voltage/current usages the footprint should be chosen? For this project the output wont be higher than 5V1A but I would like to learn for future projects with different values also.
How does that help with understanding which SMD footprint / volume to choose?
I know that I need 22uF and 10V. But I dont know when to choose 0603, 0805, 1206 or whatever footprint
Well I know that there exists capacitors of 22uF, are rated for 6V and come in the footprint of 0603. But there must be a reason why many people have all their resistors in 0603 but choose a capacitor with more volume than 0603. And I just want to understand if there is a guideline which volume to choose for a certain energy
The size of the footprint will depend on how the component will be mounted.
If it is by robot assembly it may be small.
But if it's manual, my experience is that anything lower than 1206 makes it difficult to weld.
You can choose any of those. Choose what you are happy to work with. Some people find larger sizes such as 1206 easier to handle/solder.
The volume of the capacitor does not affect the energy stored.
The energy stored in a capacitor only depends on the capacitance, and the voltage it is charged to.
Energy = 1/2 * C * V^2
I dont know man...
The bigger the footprint / volume of the parts is, the more heat they dissipate, the higher the wattage rating is, the more energy they store.
If you look at the IC I'm using you find a lot of pictures. All of them have tiny resistors but MUCH bigger capacitors. There must be a reason for that.
Usually bigger volume SMDs have much different ratings other than their main value. For instane a resistor of bigger volume can handle much more watts, dissipates more heat and so on. A bigger diode can handle much more current. I dont think you can just choose the smalles components only if you can manage to solder them.
I saw a lot of pictures of the same blue Aliexpress/Ebay step-up module with a potmeter on it. The rest of them had sometimes slightly bigger caps or the same size as the resistors
You won't know why some designers choose the bigger size. I do sometimes when i want to route a PCB trace through the gap. That's easier when i put a 1206 on the board than a 0805.
And I don't want to handsolder 0603 or smaller, unless it's a repair on an existing PCB.
There is a weak relation between voltage rating and size. You cant have a 35V rated 0603. 0805 is minimum then (at Digikey).
Another reason to choose a certain size is price and availability. Like during the Corona shortage you had to design with what you could get. Even redesign to another MCU.
In another forum (I dont know if its allowed to link it here) they say that the smaller packaging SMD capacitors lose their capacitance much faster than the bigger ones once voltage is applied.
But still, I will try to make my pcb with the 0603 then and see how it works.
Not true.
If all you need is a 22uF 10V ceramic capacitor, then the size does not matter.
Capaciors with higher voltage ratings usually last longer(less change in capaticance)
If you need a specific ripple current rating then the size may matter.
