Help with voltage regulator

Hi All

I'm looking for a low drop out voltage regulator to put out 3.3v 800mAh, I've been looking around UK-RS and I've come across a SPX1117M3-L-3-3 which, well... I will admit that I'm still trying to get the hang of data sheets so here's where I need your help :slight_smile:

Here's the link to the page

The specifications on the front page sounds like it's the right regulator for me, but the data sheet just blows my mind lol.
Will this regulator take my 7v input and output 3.3v @ 800mAh?

I would appreciate any help you have to offer. And fingers crossed I might get a better understanding of how to read & understand data sheets :slight_smile:

Why do you need a low drop out regulator to take 7V to 3.3V?

Having said that the 1117 is not low drop out (except in the marketing sense), good
LDO regulators are 0.25V or so, not 1.1V. Its a good general workhorse, I use
loads (its good enough to take 5V down to 3,3V nicely).

Bear in mind if you want the full 0.8A from 7V, then the dissipation will be 3W which is
beyond the ability of a surface mount regulator to handle(*), it'll just overheat and
shutdown. Something in a TO220 package with a medium sized heatsink is needed.

Or a DC-DC converter.

(*) except on aluminium PCB.

Hi MarkT

Sorry, I doubled my voltage because I was thinking of using 2 x 3.7v batteries to get it within the 1.1v dropout range. So basically this regulator won't work if I input 3.7v because it needs a minimum of 4.4v to output 3.3v, and if I go with my initial 7v it will overheat?

I have ordered a a few 1117 in a TO220 package because I thought they were low drop out to, but I've just realised they are the same as the above, so I'm no better off :frowning: I say that because I am very limited with vertical space I could just about get away with using a TO220 but theres no way I could fit a heatsink as well.

DC-DC converter is looking like a good option now, but can I get / build one that will give me 0.8A @ 3.3v?

Edit:
DC-DC converter is looking like a good option now, but can I get / build one that will give me 0.8A @ 3.3v and have an input 3.7v or less (like 3.0v)

You need something like that: buck-boost dc-dc converter

If I was desperate and had no dc dc switching regulator.

I'd stick with what you got and use a nice power transistor to do the work rather than the reg on it's own... if you have a transistor capable of delivering ..

Ps 800mah the h refers to "hour" eg the battery has a capacity of.

800ma is the current draw.

You can put one 5V and one 3,3V regulator in series and connect it to 7V. Than there will be about 1,5W of heat on each regulator and I think, that you needn't put heatsink on it, it should handle the heat.

lilsancho:
if I go with my initial 7v it will overheat?

Probably. 4V at 800mA is over 3W of power it has to trow away.

You could add a heat sink to cope but think: Do you really want to be running a 3W heater from your precious batteries?

A power converter will be more efficient and pay for itself very quickly.

cjdelphi:
Ps 800mah the h refers to "hour" eg the battery has a capacity of.

800ma is the current draw.

Try again, the symbol for ampere is 'A', not 'a'. mAh and mA. mAh is not SI though, physicists
use C (coulomb, an amp-second, or kC (about 0.28Ah).

You could add a heat sink to cope but think: Do you really want to be running a 3W heater from your precious batteries?

Well it's going to be handheld so it might be a plus in the winter :slight_smile: but your right, not a good idea.
I am leaning towards a power converter because it seems more practical, I won't be limited to my 3.7v battery as i could use 2x AA batteries. - to enlighten you all bit, I'm building a board that will fit inside an xbox 360 controller shell, essentially I'm building my own arduino xbox controller from the ground up using smd components (just so I can fit it all in).

I'm planning on using an Xbee for the wireless which is why I'm looking the higher mA draw, because I want the xbee to be upgradeable. e.g. from the series 2 2mW (40mA) to the 60mW (295mA). Or better still a different type of radio system that may require a little more.

So far its 1 step forward and 2 back, my current problem at the moment as you all know powering it.

Does anyone know of any good tutorials for building a DC-DC Converter that would suit my needs or a good chip to start with?
Do bear in mind that I'm learning as I go here :slight_smile:

Are you going to make your own PCB?

Are you going to make your own PCB?

Yep, this is the second version of my build. I toyed around with a design last year that worked pretty well - it was basically a pcb with an arduino pro mini bolded on to it, very crude :slight_smile: so now I'm just going to build the board from the ground up but with a few modifications to help make it more useful.

I'm documenting it as I go so I can put it up as an instructable - I've seen one design going around but the creator didn't want to share any designs so I figured I give it a go (and its going well so far :slight_smile: )

Using a genuine controller via a PC is ok, but a standalone controller will be even better!

MarkT:

cjdelphi:
Ps 800mah the h refers to "hour" eg the battery has a capacity of.

800ma is the current draw.

Try again, the symbol for ampere is 'A', not 'a'. mAh and mA. mAh is not SI though, physicists
use C (coulomb, an amp-second, or kC (about 0.28Ah).

Nice useless post as I was refering to "h" I have no intention in making my posts longer to type on a mobile phone.

It is probably cheaper and easier to buy a premade module.

Have a look at this one:

Whether your LiPo is above or below 3.3 v, you will always get a steady 3.3 volts out.

Here's another one that has a LiPo charger built in as well:

It only puts out 200ma at 3.3v though. (keep the low powered Xbee)

I do not know why you need a 60mW Xbee in your Xbox controller shell unless you want to control it from several hundred metres away?!?