I decided to make a shield for a project and I will need to make 5V and 3.3V from a car battery (12V).
I have two desing in mind and I am not sure if one is worse than the other.
First idea - Using 2 DC-DC fixed switching regulators to generate 5V and 3.3V directly from 12V.
This is just very simple but I don't if this will cause problem as they are switching and maybe interfering with the 12V?
Second Idea - Using a DC-DC switching regulators to generate 5V and then a buck converter to go from 5V to 3.3V
I have seen this on the Arduino Giga. Is it less noisy or maybe more efficient?
Either will do. Don't be concerned about the switching interfering with the 12V, the input capacitors will take care of that.
Some thoughts:
Check the data sheet for the converter for 3V3 from 5V, will it work with 5V in? (I don't know, I'm just suggesting you make sure)
How much current do you need at 3V3? I'm thinking if under maybe 75mA or so you may as well use a linear LDO regulator from 5V.
If you need a lot of current at 3V3 then feed the 3V3 converter from 12V so its current does not add any load to the 5V converter.
I don't believe the LM2675 data sheet suggests all those capacitors, when I use them I have one capacitor on the input and one on the output.
You talk about 'switching regulators' and 'buck converters' as if they are different kinds of thing. A buck converter is a kind of switching regulator.
In general I support the first version. Letting one regulator be the load of another regulator raises a question about interference between the 2 regulators. Okey, there are caps but...
Good point. I was planning to use the LM2678 at first and I switched to LM2675 because the availability at JLCPCB. That's where all the caps are coming from, I forgot to look at the LM2675 datasheet.
How much current do you need at 3V3? I'm thinking if under maybe 75mA or so you may as well use a linear LDO regulator from 5V.
I haven't calculated how much current I will need ultimately. The hungrier chips are powered by by the 5V I think. I would have to read through the datasheets and compile the current usage and make sure.
also, I said buck converter but I think it is an LDO in the second example.
I think will stick with my first design as it look simpler to me and there is no drawbacks really. However I need to change the exact component I select because the one in my schematic is not the same as the first I intended to use and the specs are not the same.
As long as you're using switching regulators. If you were using linear regulators, getting 3.3V from 12V would be overly wasteful of energy (for the 5V too, actually.) However, using a linear regulator to go from 5V to 3.3V isn't too bad...
