Powering my Prototype PCB - onboard Regulator or External Supply?

Question

I'm laying out my PCB for the first prototype of my project. The board is relatively simple as it only consists of two CPLDs (Altera 240Z Max V) and a AT Mega 128L. I am now thinking of laying out the power section but am wondering if I should just power it from an external supply. The supply meets my requirements (3.3V and 1.8V rails) and has enough juice to run the board (2A @ 3.3V and 1A @ 1.8V).

I would really prefer this approach but am wondering if this is a good idea. It keeps the simplicity as it means I don't have to test and pick voltage regulators at this stage. The power requirements may even change in the future depending on my CPLDs. So my questions are:

1. Are there any drawbacks I should be aware of of this approach? Should the power section be always close to the relavent chips?

2. Based on the advice from my previous question, I feel I have sufficient decoupling now (0.01uF and 1uF caps on every power/gnd pair with very short loops). But should I include a large capacitor (say, 470uF) and apply power straight to that?

Answer 1

Personally I usually power my prototypes from my bench power supply (it's an old ATX PSU). As you say, it saves picking a VReg at this stage.

Yes, you should have a nice meaty capacitor where the power enters the board.

On the times where I do want to have an unregulated power input to my prototypes (say a wall wart) I usually over-spec the regulator to use, so that I don't have to worry about the power requirements changing. I often don't worry too much about efficiency at this point though, and very often go for a linear regulator with heat sink where a switching one would be far better, but for a quick-n-dirty prototype it's not too much of an issue.

It is quite common for the power supply portion to be on a separate board any way - makes for a more modular system that is easier to design / redesign. Also if you are involving high voltages (well, mains - I know some of you don't think of that as "high") keeping it away from the main board is best as it means you can have the board running and not worry about handling it live.

Answer 2

Why not lay it out with pads/holes for say 7805 form-factor regulators even if you plan to power it from a bench supply at this point? You don't have to populate them, it won't cost any more to have the boards made and you will be ready to experiment with regulators if/when you want to.

Answer 3

One thing you generally have to worry about with programmable logic (and chips with multiple voltage supplies) is supply sequencing: the overall timing of voltage rails coming up to nominal when power is turned on. This can be an issue when the start-up speeds vary (i.e. linear versus switching), which is something you can see easily on a scope (use one-shot triggering).

However the MAX V device handbook, chapter 5, says "MAX V devices are designed to tolerate any possible power-up sequence." Since there are no specific requirements, you don't need to worry about this particular aspect in your design.