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LMR16010: Capacitor Selection

Nino Abundes7
Prodigy 80 points
Part Number: LMR16010

Hello,

I am trying to design around this part, and the datasheet is slightly confusing about how to select input and output capacitance:

Output: In section 8.2.2.4, an exercise in capacitor selection is done and it seems to calculate a minimum output capacitance of 15.4uF, but at the end of the section, it summarizes by saying the most stringent criteria sets the output capacitance at 47uF. How was the value of 47uF reached? There is no minimum capacitance specified elsewhere, but I didn't know if there should be for compensation reasons or something else? 

Input: In section 8.2.2.6, the datasheet recommends a decoupling cap of 4.7uF to 10uF. It also says that additional bulk capacitance may needed (our design will be much further than 5cm from the source). My question is how much bulk capacitance is actually needed to convert 28V to 5V? I would think there is enough of a difference that even if the voltage on the input were to greatly sag, there is so much headroom that it should not matter. 

  • 0
    TI__Mastermind 23845 points
    Hi,

    For the output capacitor, you can use equation 12 and 13 for finding the required value based on overshoot or undershoot that you allow on the converter. Keep in mind that this is ideal value, that means that there is voltage derating associated with it and some temperature dependencies. Therefore i think the datasheet is trying to be on the safe side and use 47uF capacitor to account for derating due to voltage and temperature.

    For input capacitor, the decoupling capacitor is more important (4.7uF to 10uF). The bulk capacitance is used only for damping purpose if you have a long lead length to your power supply because long lead length is considered inductive. Therefore you need the bulk capacitance ESR to dampen the overshoot due to LC ringing that could be happening. As far as value, i would not think that would be that critical, 22uF - 47uF should normally be ok.

    Normally we put just a place holder on our EVM PCB footprint and not populate this bulk capacitor, we use it for just in case the user is using a very long lead to our board we then populate this bulk capacitor.

    Thanks
    -Arief