I’m designing an LED array driver that will operate on battery power in the field and USB when connected to a computer. I plan to use 4 AA batteries, giving me 5.5 volts or so and the USB will supply 5.0 volts assuming the computer doesn’t malfunction. Is there any need for a voltage regulator? Maybe a DC/DC converter? I’ve tested by breadboard prototype with with both and they run fine, but it hasn’t been prolonged tests, or “in the elements” Any input would be awesome.
How about seeing the actual circuit you are planning on using?
Here’s the current schematic I’m working with in Eagle. I’m currently working on a design revision that among other things is getting rid of the power jack and voltage regulator seen in the bottom left. I have limited room on my PCB and am worried about hand soldering a crowded board. Are there any general guidelines for when you need a volt reg and when you don’t or could use a DC converter instead?
If you switch to a 74HC595, I think all the parts there will accept a wider voltage supply range.
Then you can get away with diode ORing between all the potential 5V supplies, choose Schottky diodes for the USB and 5V regulator, but a normal diode for the batteries, which will match all the supplies when tied together at just below 5V.
That’s good new as I’ve actually already switched to the 595’s on my breadboard proto. I didn’t know if they’d be able to handle the load at first and figured I’d need to use an “LED driver” shift register. Losing the 5 volt reg and the power jack also clears up my need for larger filtering caps!
I'd be willing to bet that even if you included a fair amount of filtering, decoupling, and "bulk storage" caps, once you get to turning on a bunch of LEDs, the spikes in the power rails will clobber the PIC and cause it to reset.bveenema:
Losing the 5 volt reg and the power jack also clears up my need for larger filtering caps!
If the LED drive is that noisy, then just separate that supply from the PIC, just another set of diodes.
One thing to watch is to make sure even with a fully charged battery when the USB is connected that power is being drawn from the USB. You’ll have to measure the drops under various loads, it might end up being 2 diodes in series on the battery.
You could also get fancier with MOSFET switches, but diodes would be fine.
once you get to turning on a bunch of LEDs, the spikes in the power rails will clobber the PIC and cause it to reset.
I’ve built a nearly identical circuit on my breadboard, it uses 595’s instead of the HEF4794’s and it works fine. I’ve had zero issues with the pic reseting itself. I can’t think of any reason this would change when I switch from the breadboard to the PCB but then again, the only other PCB I’ve made was just one to hold my LED arrays.