3.3v

nos_slived · Post by **nos_slived** » Wed Jun 22, 2005 11:55 pm

It will probably be August by the time I get the samples, because I can't order them until late July. I also need to start working on my Ni-MH and Ni-Cd chargers, but I am still trying to figure out some of the components.

rawls · Post by **rawls** » Thu Jun 23, 2005 9:32 am

Why not use an LM651 (here is the data sheet)? On the data sheet is an example circuit for how to wire the thing up for 3.3V off of a 5V supply line. As long as you run more than 40ma, you have a better than 90% efficiency and you can run it off of your regulated 5V supply. Not only that, but these guys are like $4 from Digikey. You have to buy a few extra parts (like 6 capacitors, a resistor and an inductor), but you are still looking at probably less than $15 for an ultra high efficiency regulator that (because of its high switching frequency), shouldn't take up too much room. And then thing can supply up to 1.5 A.

nos_slived · Post by **nos_slived** » Thu Jun 23, 2005 11:57 am

Actually you can't run it from your reglated power, because the 7805 has a 1A max output, so you will end up frying it when you try to draw that much power. Also, the LM317 requires less parts for it's circuit. It only requires 2 resistors, and it can also supply 1.5A, and is pretty efficient. You also can't get any free samples of the LM2651 to play with. Yes, the LM2651 does have the "Live Simulator", which gives you part values, but if you order these parts from Digikey, it will probably cost about $15 parts + $5 fee for being below $25 minimum.

rawls · Post by **rawls** » Thu Jun 23, 2005 2:33 pm

Well, running the LM2651 from a larger voltage (like your batteries, which can source a lot of current) will give it better efficiency anyway. Or, put a better 5V regulator (7805s aren't good for battery operated stuff anyway, especially in a power muncher like an N64).

And you say that the LM317 is "pretty efficient." I don't know what this means, but I don't think it will be that high. I can't find any efficiency numbers from the datasheet, but in an application note a technique to use an LM317 as a switching regulator is discussed and even this has just 60-80% efficiency. I'll bet you cannot get a whole lot higher than 60% efficiency from the LM317, especially if you run it off of 12V (it is linear, after all). Now, if the current draw is small, doesn't matter too much, but if it is big (and you seem to imply that), then, for purposes of battery life, this can be a problem.