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, 02-20-2024, 05:39 AM

Hello I need a little help from an expert to understand how to choose an input capacitor for my design. I am designing stm 32 base + GSM base design. I use 12 volt to 5 volt SMPS IC to power the board and use ldo to deliver power to MCU and GSM modem ic. I place one close to the SMPS IC and other little bit for away from SMPS IC close to the GSM module. The ldo I place near to the GSM module, according to the data sheet of that ldo it requires 10 UF capacitors for input power. Is it okay if I use that value or higher value capacitor like 100 UF considering the distance of this power source and ldo to have proper function of that ldo?
The ldo that I use is LD39200

Robert Feranec , 02-20-2024, 07:12 AM

If I would be designing this, I would use two separate power supplies, one for GSM and another one for MCU. I have seen number of times how GSM can make problems on power rails - in your lab it may work, but there may be some random and occasional peaks which can make problems for MCu e.g. reset it

Mini , 02-20-2024, 04:58 PM

Robert is correct, use separate power supply for GSM if possible. The problem is GSM takes 1-2 amps in short period of time. And it's not only max current your LDO can handle, it's also about reaction speed. If your LDO is slow it won't react fast enough and your voltage will drop. To actually know that is your LDO good enough and how much capacitance you need is to do real life test. If your LDO can't react fast enough you simply will need a lot of capacitors on the output where will be enough energy stored.

Mini , 02-20-2024, 05:01 PM

You can also look for some reference designs and see what others have done.

QDrives , 02-20-2024, 09:13 PM

Q=C * dV; Q=I * t
Lets state dI is 2A, Fbandwidth LDO is 50kHz -> 20us. Q = 2[A] * 20[us] = 40[uC].
Lets assume dV of max 0.1[V] -> 40[uC] = C * 0.1[V] -> C = 400[uF]
Do not forget DC and AC bias when using ceramic (ie. X?R) capacitors.

But as answer to your question - No it is not a problem to have more capacitance on the input.

, 02-25-2024, 03:48 PM

Thank for the help I will use another power supply for the GSM module