Switch Mode Powered Arduino - Prototype built, looking for advice?

[Zigadenus]

Hi Guys,

I'm looking for some design feedback on a prototype I have already designed / assembled. My goal was to make a switch-mode powered Arduino copy, with very low quiescent current and selectable high amp 5V pin + 5V USB power supply, for remote power and occasional high power radio transmissions (though its a flexible platform).

It's more complex than anything I'd made before but great experience for improving my Eagle CAD etc. Mainly I'm looking for any obvious design defects / advice as I'd never made SMPS before, but troubleshooting advice of problems is always welcome. I've attached pics and Eagle files for everyone.

I settled on:

   1. ATMega1284 microcontroller, requires some modification to boards.txt but a relatively well supported environment, Arduino compatible and extra connections / space    available for the supplemental kit on the board
   2. 3.3V / 8MHz as the base PCB voltage
   3. On board voltage divider to measure input voltage by the chip
   4. On board micro-USB for datalogging
   5. Standard Arduino widths for shield compatability
   6. ISCP header for programming
   7. LM53603 SMPS, automotive specced and allows 3A output and 24V input, very low quiescent current 
   8. TLV1117lv 3.3V LDO linear regulator (input from the 5V SMPS) for on board power, also low quiescent current
   9. USB connection, allows external USB devices (RPi / GoPro / transmitters) to be connected
   10. AOP605 transistor to switch the 5V supply on or off (will replace with something more modern)
   11. Sparkfun FTDI pins for external Serial connection and potentially programming without using the ISCP headers

As it stands:

   1-8: All seem to work as planned, 5V and 3.3V on the pins, programmed with ISCP, MCU and SD read/write functional
   9. I used a male USB connector, because that's how smart I am! However female-to-female adaptor shows voltage drop to 4.3V (and lots of noise when a load is applied. I suspect because..
   10. I clearly cocked up the connections to the AOP605, it was my first time using transistors like this. My goal was to replicate the design: https://electronics.stackexchange.com/questions/103678/proper-usage-of-a-mosfet-to-control-usb. However, mine doesn't switch off on low voltage, and the input 5V gets noisy on load as well. I'll try and figure this out on the breadboard because I didn't have the chip when I made the design but pointers very welcome.
   11. I swapped TX/RX on the design and didn't connect DTR to reset with a capacitor. When fixed with external wiring I can use Serial connection in and out as planned. However, trying to program gives a different resp=xx error message each time I try.

Any advice from eyeballing the design, obvious mistakes or advice what I can do better on the board?

Thanks!

[westfw]

Any advice from eyeballing the design

certain shields use the ISP connector for SPI data, and won't like that you've moved it from it's standard location.

   9. USB connection, allows external USB devices (RPi / GoPro / transmitters) to be connected

You mean, just for providing power to those devices?

I like wider traces and bigger clearances, when I can get away with it.  Even your thick power traces look a bit thin for 3A!

You have a 3.3V power plane on the top layer and GND on the bottom?

You might want to put in tie points for the unused pins that are easy to do so.  No one ever complains about having extra signals be "somewhat available"

The switching converter layout looks a bit "big" compared to what I see in datasheets; but I've got no actual experience.  Does it work?  It might be worth stress testing over a range of input voltages and output currents, to make sure it behaves as well as you hope.

[Zigadenus]

Thanks for the reply and the great suggestions, I thought I'd sunk without trace there!

certain shields use the ISP connector for SPI data, and won't like that you've moved it from it's standard location.

Didn't know this, I can move that back.

   9. USB connection, allows external USB devices (RPi / GoPro / transmitters) to be connected
You mean, just for providing power to those devices?

Yes, there's no data link through the USB port but the 5V input can be switched on

I like wider traces and bigger clearances, when I can get away with it.  Even your thick power traces look a bit thin for 3A!

I used an online calculator for the widths, but I should try and test at full power to see what happens. I don't have a fancy load tester like Dave, are the special resistors (or just low value) that you can use to create high currents?

You have a 3.3V power plane on the top layer and GND on the bottom?

Other way around

You might want to put in tie points for the unused pins that are easy to do so.  No one ever complains about having extra signals be "somewhat available"

Good point, no reason the 5V power can't be activated externally if needed too.

The switching converter layout looks a bit "big" compared to what I see in datasheets; but I've got no actual experience.  Does it work?  It might be worth stress testing over a range of input voltages and output currents, to make sure it behaves as well as you hope.

Ah yes, well 'works' turns out to be a somewhat conditional affair. My first time making one. Above 100mA the 3.3V reg goes into dropout (I assume due to the noise), which creates a 4.8V output on the 3.3V pin! I looked at chokes to try and lower the noise, but it may just be I have to me much more aggressive about the component distances around the switcher. I knew it was an important thing but clearly I didn't make it tight enough. Would it be usual to have a choke on the output of an SMPS regardless?

Thanks again for all your pointers.

[maginnovision]

As it is your components are sort of everywhere. Following the datasheet layout is usually pretty safe. As for a choke, that's part of what the output caps and the inductor are doing creating the low noise output but with everything everywhere it doesn't work.  Vias from switcher to inductor are usually frowned upon. Do you have a way to shutdown switcher in the case of usb power? looks like usb power and switcher power will tie together which isn't good. also for a switcher that size I'd probably use 0402 passives so they fit around it better and use a pour to connect inductor and switcher, get output caps closer to the inductor too. you'll probably want a fuse of some sort too for the 5v connections considering it's an arduino board and you don't want to kill anything. Alot of the time you would just use diodesfor arduino type stuff. So diode after output caps and after usb 5v in to "select" power source. The control solution you're using seems way pricier and complicated than needed.

[Zigadenus]

so for a switcher that size I'd probably use 0402 passives so they fit around it better and use a pour to connect inductor and switcher, get output caps closer to the inductor too.

Great point! I'd seen the datasheet layout and couldn't get everything to fit because I always use 0805, time to bite the bullet and go smaller then I can get closer to the ideal design, and eliminate the swithcher-inductor vias and use whole pours to connect things (which is also closer to the datasheet design).

Alot of the time you would just use diodesfor arduino type stuff. So diode after output caps and after usb 5v in to "select" power source.

That's a neat solution idea, thanks.

The control solution you're using seems way pricier and complicated than needed.

Do you mean the choice of switcher? I wanted very low quiescent current and wide input range, I used Digikey to find the switcher. On standard MCU operation I have total power draw of 4mA vs. 40mA for the standard Arduino. If there is a cheaper switcher or layout out there I'd be interested to know.