Good idea/bad idea, given that the device works fine under 4.5v?

-Colin

It depends on two things....

(1) Will the 5 volt stuff be happy with 4.5V? Normal TTL logic is speced to run from 4.5 to 5.5 volts so if the voltage doesn't drop lower, it will probably work fine. I'm assuming that's something like what you're doing. Most parts that want 5V will work fine with 10% above or below the rated voltage.

(2) Can your Vref supply enough current? A resistor divider will not give you enough current to do much real work. Luckily there is an easy work around. Just use an opamp as a unity gain follower on the Vref and you should be good to go, up to the supply capacity of the opamp output. This configuration is often known as a synthetic or artificial ground.

I say give it a shot under worst case scenario, and full speed ahead!

You might run into problems with too much voltage, but too little will only cause funky operation if anything.

Take care,
-Peter

If you want a REALLY cheap and cheerful solution, wire up a 3V3 zener in series with a silicon diode to drop 4V (the silicon diode can double as a protective device for the rest of the circuit).  A certain amount of current (usually a few milliamps) needs to flow through the zener for it to do what it says on the tin AND you'll probably need a RC filter after this arrangement as zeners can create a little noise, but I've used this in the past when I've needed to - crude as hell, but it works! (well, for most non-critrical things)

All in all you might as well use a regulator, they don't take up much space.

It would help if you gave a little more info in your posts Colin.  No offence, but they're a little cryptic sometimes  

Right peter, it's a TTL device that wants between 4.5 and 5.5v.  I'll try to be less cryptic from now on  Full speed ahead!

-Colin

Couldn't you use a voltage divider to supply 5v instead of 4.5? Like instead of a 10k - 10k you'd have a 8k - 10k if I'm correct.

watch ye out: (if I understood your plans well...)

if there is extra drain going from the divider`s Vref (be it 4.5 or 5V...),
this will upset the balance of the 2 formerly equal resistors,
with the result, that the "southern" resistor which is being parallelled with the TTL-circuit`s load, will look much smaller than the "northern" one;
i.e.: Vref will not be Ub/2 anymore, but much lower, too low to bias the stuff you have hanging to it for that purpose;
in fact, the former Vref will probably fall lower than needed for the TTL-stuff.
(except, of course, when you use a rather low-impedance divider, like say, 100 Ohms).