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Switching regulator for dev board

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Blueteeth

Well-Known Member
Hi,

I don't like to waste topics/posts with 'pointless' questions, but I have googled this one, and the number of results from maxim/national semiconductor are just overwhelming, so I hope someone could help me narrow down the search.

I'm designing a dev board for PIC (and AVR's :D) and I would like to have a switching buck regulator to reduce the profile of the voltage regulator section (or area if its horizontal), and to negate the need for a bulky heatsink. Since its a modular design, any 'modules' attached will draw their power from the boards supply, so 1-1.5A max. is needed. A simple 7805 is out of the question here.

Now, What I would *like* is the ability for it to be powered either by external DC (6-?v) OR USB power. And outputs 5v or 3.3v depending on a jumper setting/switch. The problem is, that, USB doesn't always provide 5v exactly, so it could be possible that the regulator would have to 'boost' the voltage ever so slightly for 5v apps, or at least pass it through. So I guess the input should be from 4.8v (ish) to say, 12/15V.
I realise I could make my life a lot easier if I just have three jumper settings:
external 6-12V to regulator - 5v...with a jumper setting to switch this to 3.3v. And USB direct power, with a setting for 'USB direct or 3.3v reg'. That would require two regulators, one for 5v and one for 3.3v. Ok, so thats 4 settings, 2 for input, and 2 for output. 3.3v is needed, because level shifters (bidrectional) seem to be pretty expensive.

As always I tend to vastly overcomplicate things, so any advice would be helpful.

Regards,

Blueteeth
 
**broken link removed**

Why are you worried that your 5V supply is 4.8V?

A typical 5V regulator has a tollerence of 5% anyway so the output could be between 4.75V and 5.25V.
 
There are a number of regulator IC mfgs that have what you need. The most important rating you have here is the max voltage rating of 15V. You first need to narrow your search to those ICs that can convert from 5V to 15V. You also want to get a regulator IC that has 100% duty cycle capability. Another nice feature would be feed forward voltage compensation that would automatically compensate for the large input voltage variation so stabilizing the regulator would be much easier. Modifying the output voltage would be easily done by changing the impedance between the FB pin of the regulator IC and ground. The feedback resistor should remain constant - modify the output with the set resistor. Some of the ICs may be able to get bias from a wide input range of 5V to 15V. Others may require a constant 5V. A constant 5V isn't a problem - just use a sot23 pkg'd LDO to create the 5V bias. You may need to do some dickering with the bias circuit if the input is below 6V.
 
something like this?

https://www.dimensionengineering.com/DE-SW0XX.htm

problem with a DIY switcher is that it most likely will be larger than the linear it's replacing.

for my breadboards, I just use common off the shelf external switchers (switching wall-warts) - they're filtered up the wazoo, certified by all sorts of regulating bodies and best of all cheap, costing less than I could ever build one for. plus they have all sorts of safety circuitry and feedback/compensation circuits for handling input and load transients, shorts, etc.
 
One of the OPs design constraints is that the INPUT can be as low as 4.8V. Perhaps the regulator would suffice if there was logic incorporated that would bypass the regulator and turn on a pass FET for input voltages between 4.8V and 6.5V for an output of 5V, but then there really is no regulation in that range.
 
good grief ... this is for a dev board? why such strange demands - is wall outlet power unreliable or in short supply?

some of the OP's requests are mutually exclusive

for example, 1.5A current with USB as a power source?

a simple SPDT switch and a 3.3v LDO is all that's needed ... switch either routes USB Vbus line straight into the board's power bus, or USB Vbus straight into the 3.3v LDO, which outputs onto the power bus.

It'll take a pretty advanced design to get an eight watt smps to take less space an LDO linear. Especially if you want very clean power with short circuit protection, good transient response, tight line regulation, etc. Sure a chip from Maxim, a diode and a coil and a few caps gets you an SMPS, but it's not very robust, and has minimal to no protection.
 
First off, USB places relatively stringent specs on its 5v power rail. You would have to have a pretty demanding application to have any real problem with it, so I'd suggest you stop and think about whether it's really as much of a problem as it seems.

have you looked at National semi's WEBENCH site? you plug in the specs of your desired converter and they pretty much hand you a design. I plugged in 4.5-15v as input range and 5v@1.5A as output and it suggested the LM2587-5.0 regulator chip.

If you want it real simple, these types of 'all in one' regulator chips from national/Linear/etc are nice, with internal switching transistors, simple reference designs in the datasheets, and not too many external components. They're generally not cheap though. Switching converters are generally something you can't just toss together, so if you want to do it better you're probably going to have to learn a reasonable amount about designing them. I'd suggest looking around for some info on the basics, and grabbing linear's free simulation software SwitcherCAD which comes with quite a few example switching converters that you can play with.
 
Hi and thanks for the replies.

Obviously for 500mA+ apps, external power would be needed, as the USB can's do over 500mA, and probably won't even go over 400. I have a few 'wallwarts' switching and otherwise, and a 5v one would be convenient...but most I have are 9v, 500mA max. A 7805 would disipate 2W at full load (limited from the PSU) and would require a heatsink, ergo, profile.

That is, my board is of a stack design, so a vertical TO220 is too high, and a horizontal one..well...the heatsink would take up too much space. Its a double sided board, so a SO-08 underneath, with a few resistors and small caps is allowed. Any inductors, or larger caps would have to go on top though.

Hero999: good point, didn't think of that, so I guess the regulator doesn't need to take 'less than' 5V, I could just pass the USB power straight through for 5v, and use a 3.3v reg for, well, 3.3v :D

Outtolunch, I'm googling now, great response there, would be nice if I could get it down to a single IC.

Just DIY, good point, but this is for my own dev board, I'm bound to lose a wallwart and have to use another one, (say 9v?) and the 5v ones I've seen get expensive when the current gets to 1A. I do happen to have a switching reg of 10v @0.75A, would be nice to get 1.3A out of a SMPS at 5v.

Evandude: Excellent point, this is what i've been umming and arring about...is USB power good enough? well, it seems after your post and research, yes, it is :D I'll just bypass the switching reg for USB apps, and have a jumper that selects between that and the output of a 3.3v reg.

I realise that 1-1.5A seems like a big deal, after all, for motor apps, I'll have a seperate power supply for a 'H-bridge module'. But I have a few LCD's whose backlights require a healthy 600mA, and they just look crap at 350mA. I'd say 95% of all I'll use this dev board for won't need more than 50mA, but its nice to have it there as and when I need it.....plus I've never build a SMPS before, like this dev board, its a learning experience as well as a handy thing to have.

Well advice taken on board (punn intended) I think I'll look for a relatively 'cheap' switching reg for 6-12V, so I can use more than 500mA from my 9v wallwart and any PSU I have right here, a jumper for USB power, and one for 5v/3.3v. I could have gone down the LM317 route, sure, but size and neatness, and the 'cool' factor :D
Just for kicks, I may look into an automatic 'USB power' switch, but that might be going too far, this is just a small 80x100m dev board after all.

Thanks again guys,

Blueteeth
 
since this is a stackable design, why not have your power supply as one of the layers, then you have ample room for a horizontal heatsink. this also gives you flexibility room for various connectors etc.

here's the idea ... get the LM1086 (LDO reg, wastes a lot less power than a 7805) in the TO-263 package, and solder the tab to the circuit board ... now your entire board is a heatsink. if you're making a two layer board, one layer can be supply and one layer ground, the tab is supply.
 
Hi JustDIY,

Hmm, that is a good point...I was hoping to have the bottom board as the 'base board', which would simply have the oscilattor section, the power section, and a load of headers for I/O's. As well as a serial/USB and special purpose headers. I think I've shot myself in the foot with this one, because of the board space limitations, it'll be ridiculously compact.

Alas, I don't think I can justify using an entire stack board (costs me about 25USD each...and I've got lots of designs for them) for the power section. If needs be I'll just have to sacrifice the power requirements to fit it on the board. As I said, I do tend to over complicated matters and get carried away with 'perfection', but everything is a compromise, 3.3 isn't always necessary, and I guess I could just use an LDO for the 5v, with a 6v wall wart.

Saying that, I have seen these wonderful AVR boards at: **broken link removed**

These use what appears to be a relatively simple SMPS, with a LDO 3.3v reg. I'll check availability of the chip it uses, but it appears to push all my buttons. I tihnk I'll still go with a SMPS, as opposed to a linear reg, maybe add a SMT LDO reg on the bottom of the board as an auxiliary. I have space, although its getting crowded already.

I'll post one of my latest layout designs in the topic I had for a 'PIC dev board' if anyone is interested.

Blueteeth
 
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