(Please excuse my English)
I have two types of DC-DC step-down(buck) converter modules, and the switching frequencies of the converter modules are 1.4MHz and 150kHz each.
The two converter modules supply 3.3V power to two sensitive sensor boards. I should remove the switching noises of the converter modules.
However I cannot modify the circuits of the converter modules. The converter module circuits are fixed on PCB. So I want to append some circuits to V-out(3.3V) of the converter modules.
What are the simplest circuits for my purpose?
Best Answer
1) All DCDC converters will produce some ripple (switching noise) at their output. It is unavoidable and inherent to the way switched DCDC converters work.
2) Before you continue, read and understand 1) and if you understand 1) correctly then your conclusion should be that you will not be able to completely remove the switching noise. So you will have to determine how much switching noise is acceptable. So how much lower does the ripple need to be, for example 10 dB lower at 150 kHz or 20 dB or 40 dB? Note that if you say: "But of course I want 40 dB!" then I'll say, OK, that will cost more (more complex filter, more expensive components) etc. but also: Would you like 80 dB suppression? And if you then say "yes please" we will go on and on and your design will be infinitely expensive and have way too much suppression. So figure out what is really needed. Yes, that might not be easy.
3) Once you know how much switching noise is acceptable you could choose to filter the output voltage of the DCDC converter. A filter like this could do the job:
Another option would be to use an LDO (linear regulator) for supply voltage regulation. An example: a DCDC converter is used to convert 12 V to 5 V, that 5 V with the switching noise is then regulated by an LDO down to a far less noisy 3.3 V.