I need to power a board that works at 3.3V, and I have a regulated power of 12V.
I tried to use a single linear regulator like the TS2940 but the power required is so high that the heat produced was too high (melted the case).
So I decided to use a switching regulator (the LM2594) to go down to 5V and a LDO linear regulator to go down to 3.3V. I read everywhere that this is a common practice.
The switching regulator works fine but my problem is that the linear regulator does not work properly (2.8 Vmean, 1.2 Vpp).
That is because by datashet the output capacitance of a switching regulatr should be around 100uF, and the input capacitance of a linear regulator should not be greater than 10uF.
What is the best way to adapt those two impedences??
Thanks
Best Answer
In the datasheet for the MCP1700 family, I found two paragraphs talking about the input capacitance requirement.
and
Neither one says that there is a maximum input capacitance. In fact, section 5.1 says that higher capacitance values will improve AC performance.
I don't believe there is any need to reduce the input capacitance seen by your regulator.
You will probably want to have 1 uF ceramic (low ESR) capacitor immediately at the input pin of the linear regulator, in addition to the higher-value (but higher ESR and ESL) capacitors used at the output of your switching regulator.
If you need a really clean output voltage, you might want to pay attention to what harmonics of the switching frequency your switching regulator produces, and be sure to filter those out if they are above the frequencies where your linear regulator has good line regulation.