Voltage Regulators and the Prop

CassLan

I realize that this question isn't 100% Prop related, but it stems from my use of the Prop and my desire to power it appropriately, and is more of a best practice question.

The Question:

I'm building a circuit that will be powered by 12VDC power source (Wall Wart 1A).
The Prop and·other components require 3.3V, so I'm using a LM2937.
The PC Keyboard requires 5V, so I'm using a LM7805.

Should the VIN of the LM2937 come from the VOUT of the LM7805?
This is what I've been doing based on the fact that 9VDC transformers have been recomended from Parallax for the ProtoBoard.


And the second part:

Should I be placing bypass capacitors across the VOUT of the LM7805 and GND, the VOUT of the LM2937 and GND, the + of the 12VDC Supply and GND?
I'm assuming at the very least the VOUT of the LM2937 and GND, but If If thats fed from the VOUT of the LM7805 and I have a bypass capacitor on that output am I killing 2 birds with 1 stone?

Thanks in advance,

Rick

rokicki

Either way will work. In general, the total power requirements for the two
approaches will be identical; what you may want to consider is how much
power is dissipated in each regulator in each scenario. In one scenario,
the 5V regulator has the bulk of the power dissipation; in the other, it is
split, but how it is split depends on the draw you expect.

9V is recommended by Parallax because the 12V to 5V drop is 7V, so it
doesn't take much current before you get significant power dissipation
in the regulator.

For where to put caps, I would look at the data sheets for the regulators.
The LM7805 and LM2937 are both pretty forgiving, but at the least I'd
have both a electrolytic and a ceramic on both outputs. I'm not sure you
need one on the 12V input, unless the wall wart itself doesn't have one.
But always go to the data sheets first; you don't want to copy the mistakes
of others.

Mike Green

I would suggest that you use the 5V output of the LM7805 for Vin for the LM2937. This will minimize heating in the LM2937. I think you will have problems with heating of the LM7805 from the large voltage drop. You haven't discussed the current drain on either the 3.3V or 5V supplies. Assuming that the 5V supply drain will be over 1/2 A, you're talking about at least 12V - 5V = 7V x 0.5A = 3.5W of heat dissipation. You will need a good heat sink for this. If the current drain is higher, that's a lot more heat.

Whenever you have a large voltage drop across a regulator like this, you should seriously consider using a switching regulator which would avoid this problem. There are some complete switching regulator modules that are designed as drop-in replacements for the LM78xx series of regulators.

All regulators require some bypass / filter capacitors across their outputs. Most require some input capacitance if there's any distance between the input of the regulator and the power source. Different regulators have different minimum capacitance requirements. Check the datasheet for the device for the specifics. If one regulator feeds another and they're physically close to one another, then the output capacitor for the first one serves as the input capacitor for the second one.

Mike Green

Here's one example of a ready-made module:

focus.ti.com/lit/ds/symlink/pth08080w.pdf

CassLan

Thank you both, I will read the regulators Datasheets.
I will also check the current draw from each regulator to take into design considerations.

Rick