# Power management for a bus/self powered USB device

I'm building a development board similar to the Arduino, with similar power sequencing requirements. It has both DC and USB power support, and it should prefer DC power. The DC input needs to support a wide range of input voltages and regulate down to 5v. I also need a 3.3v regulated output regardless of the power source.

My current solution is rather klunky. I have a buck regulator for the input power, an LDO for 3.3v, and a power switch for USB consisting of an opamp set up as a comparator and a P-channel MOSFET - the circuit used in the Arduino Uno schematic for power switching. I also have a diode on the input for reverse power protection, and a polyfuse on the USB power for current limiting.

All of this works, but it's less than ideal - it takes up a lot of board space, requires a large BoM, and is moderately expensive. I'd really like a more integrated power solution, and it seems like this ought to be a fairly common task, but I can't find anything that seems suitable.

Can anyone recommend a solution that incorporates at least some of these functions into a single device?

Edit: Here's the schematic for the current power subsystem:

• As far as I know, something like this doesn't exist in a single IC. You're most likely going to be stuck 1) converting the DC wall wart power to 5V and 2) handling the switching off between that and USB power going into your 3) 3.3V regulator. You could handle all of that with three individual ICs plus a few supporting discretes... but it's not going to be get any more integrated, really. – Toby Lawrence Dec 24 '12 at 13:26
• @TobyLawrence I hoped there'd be something that incorporated most of this, since it's pretty much exactly the functionality that bus/self powered hubs and other such devices require. But even a better solution for power switching and current limiting would be good - this opamp solution and the separate polyfuse are not ideal. – Nick Johnson Dec 24 '12 at 13:28
• There are ways you could shove a lot of this functionality on your board through ICs, but in most cases, it'll be easier and cheaper to use basic components. For example, you could easily find a ORing controller with current limiting on a per-input basis, which is great... but that doesn't mean you shouldn't also want a fuse at the front of your device. (I'm not trying to discourage you from wanting an integrated solution, only compound on that sometimes the basics work very very well. :) – Toby Lawrence Dec 24 '12 at 13:33
• If the PMIC has current limiting, why would I want a PTC fuse as well? And I agree, more basic ICs may not be a bad thing - but a discrete opamp and PFET is neither as cheap nor as compact as I'd like. – Nick Johnson Dec 24 '12 at 13:38
• I'd be willing to bet most manufacturers are doing this the "hard" way: designing the switchers themselves, working the board layout over a few times to squeeze things in right, etc. If you go that way, you could definitely optimize costs and keep things relatively efficient when it comes to board space... but it's not going to be integrated. You have to pick what you care about most: ease of assembly, board space, cost, BOM simplicity, etc. – Toby Lawrence Dec 24 '12 at 13:50

You're going to be hard-pressed to compress all of this functionality into any smaller of a package. Let's go through what you're trying to accomplish:

## Regulate down from wall wart power (most likely 9V-12V) to clean, regulated 5V

This is easy and could be accomplished a zillion different ways. Current draw and input voltage is what really influences your choice here. You can easily get a small package linear regulator, but if the input voltage is too high, you start needing bigger and bigger packages to handle the heat, and you can get up to D2PAK and still be throwing too much heat. Linear regulators handling high input voltages is usually sucky for any moderate output current.

In this case, you need to step up to a switcher so you can avoid these heat issues. As far as the smallest package/simplicity, I have used the TPS84250 from TI in a design. About 14mm x 14mm of board space and 7V-50V input with 2A of output current and adjustable output voltage. They are very expensive compared to the raw components (switching controller, inductor, diode, etc) at $10 -$13 per piece in low quantities, but we're talking about simplicity here, right?

There are similar switcher designs in the TI Webench design center (output current / board size wise) that can be built for much cheaper, but then you're using more components and spending more time on layout. It's going to be a trade-off.

## Select between regulated 5V and USB VCC for input to our 3.3V regulator

There are also a few good ways to do this... mostly either using discretes (diodes) or MOSFETs. There are even some power controller ICs with the MOSFETs built in. Can't beat that for integrated/small. Again, a favorite part of mine is the LTC4415 from Linear Technology. This IC will OR two power sources for you with its integrated MOSFETs, and prioritizes one of the inputs automatically for you. It also lets your set independent output current limits for each output so you can configure the USB input to match your 500mA limit, and the wall wart current limit to match your switcher's output current limit, etc etc. Board space consumption here is pretty small.

Again, a little pricy... these badboys are like $3 -$5 in low quantities but they do make the prioritized power source requirement pretty straight forward.

## Regulate down from 5V to 3.3V

This portion is pretty obvious. Find the smallest package size with enough output current. Optimize in pricing, etc. Done.

## Other Considerations

I know you mentioned wanting to eliminate components, but don't forget that you still want system-level protection against component failures... i.e. a fuse in front of the switcher in case the switcher goes haywire somehow and short circuits. Same thing goes for USB power. Your device should be doing its best to play nicely with all systems and signals it integrates with.

• Thanks! I'm currently using a discrete switching reg based on the LMR12010, but I'm tempted to switch to an integrated solution. TI's TPS84320 costs about $4.50 in smallish quantities, which isn't that much more than my current solution. The LTC4415 looks ideal, but$3-$5 seems absurdly expensive for a power switch! – Nick Johnson Dec 24 '12 at 14:04 • Well, you're mostly paying for the fact that it's Linear :) However, Linear is really, really good at power ICs. They have great documentation and support. You can easily get samples and buy direct from them. Also, from the searching I've done... the actual priority feature is usually composed as "feed me power from whatever supply voltage is higher" whereas in this case, the LTC4415 prioritizes the input itself... which in the case of identical input voltages, is essential to how "integrated" it is. – Toby Lawrence Dec 24 '12 at 14:09 • Any suggestions that improve on the Arduino's dual-opamp-and-PFET solution without going the whole hog to one of those expensive parts, then? Also, I've seen ICs that integrate switching regulators and LDOs - are those likely to be practical here? – Nick Johnson Dec 24 '12 at 14:45 • I don't want to derail this as a shopping question, but what is your budget? Budget is going to dictate whether or not you end up using discrete parts (comparators, external FETs, etc) or an integrated solution. – Toby Lawrence Dec 24 '12 at 16:48 • Well, "as low as possible", though I realise that's not a budget. Probably about$5@100 for the whole power subsystem. – Nick Johnson Dec 24 '12 at 18:05

I'm not able to find a solid answer for what the maximum current output on a arduino (uno) board is, some are saying 500ma 5v on usb (obviously) and 650ma on the dc jack.

If that's the case, you could always go for a dual channel regulator instead. Cuts down on the bom and on board space needed.

TI has the tps70102, which gives you two adjustable outputs, 500ma on channel 1, 250ma on channel two. But it's vin is 6v max (great with usb-ish chargers though)

They also have the tps54283 (or really, most in the tps54xxx family). High vin range 4.5-28v, dual adjustable outputs with minimal external components, providing up to 2 amps. No need for separate ldo's.

• The main problem is that that dual buck converter is already pretty expensive and doesn't necessarily solve the main issue of switching between the 5V regulated from DC wall wart power and USB 5V. Selecting and optimizing the 5V switcher and 3.3V LDO is easily done... but the power management section is what he wasn't happy about / makes this harder to do when being cost conscious and wanting an integrated solution. – Toby Lawrence Dec 25 '12 at 14:07

Check out the MAX8934. I realize that it is a LIon charge chip, but Maxim may have other chips that do exactly what you want. It provides dual input selection, current limits for USB and line, 5.3V system out (pre-regulated) and a 3.3V LDO (30mA)