I am building a circuit that will run off the 12V battery in a bus. I need two supplies - 5V and 3.3V, the overall current consumption being <200mA. I have an LM7805 to generate the 5V supply and it then drives an LD1117-.3.3, to generate the 3.3V supply. The circuit I have designed is shown in the figure below.. My questions are as follows:
Is it OK to also add a 470uF electrolytic cap on the input of LM7805? I am hoping for additional protection against fluctuations in the 12V supply from the bus battery.
Is it OK to skip the 0.1uF on the input of the LD1117-3.3 since the output of the LM7805 already has a cap of this value?
The LM7805 recommends a smaller cap on the output compared to that on the input, whereas the LD1117-3.3 recommends a larger cap on the output compared to that on the input - any insights in to why this is the case? I am just trying to understand whether there are any gotchas in connecting the output of the LM7805 to the input of the LD1117-3.3!
Can the 0.33uf and 0.1uF be ceramic (SMT) caps?
I have some off the shelf power regulator boards that use the LM7805 but have a very large electrolytic cap (1000uF) on the input and another electrolytic cap (10uF) on the output. Why would they deviate from the recommended values of the decoupling caps?
Yes, no problem. 1000uF is not much bigger physically.
If they are close to each other, perhaps. Put it closer to the LM1117.
Partly history of the applications, but the LM1117 actually needs a capacitor on
the output with certain characteristics to keep it from oscillating like a banshee.
With the LM7805 the caps are not generally
necessary- they just improve performance (lower output impedance, improve transient response).
They can all be ceramics (including the 10uF), however you would need to put a series resistor on the
10uF in accordance with datasheet ESR limits. It's probably worth the extra part to avoid using a tantalum.
Most likely the 1000uF is also a filter cap. Going higher in value is
usually better with capacitors and those are really common values for an off-mains
supply for moderate current. 47uF/10V or 100uF/10V is not much physically larger
for the output cap and I would tend to use that.
Edit: Regarding the output capacitor requirements of the LM1117, below is the information from the TI (nee National Semiconductor, the original designers) datasheet for their LM1117:
So something like a series 0.5 ohm resistor should be fine. Note that a 10uF ceramic capacitor has a (negative) voltage coefficent as well as a tolerance, so you may wish to use a higher voltage rated part or a higher nominal capacitance part to ensure the 10uF minimum requirement is met, just to be completely safe.