Wow, thanks everyone for your help and input. I can report that over the weekend I tested out the bootstrapping idea I described in post #12 of this thread and it did not work. I tried the configuration both running on batteries and running on my bench supply. Instead of the expected 5.7V, the output was roughly .4V below the input voltage. I tried several different input voltages and the output was always about .4V below that. I'm not sure why the idea didn't work but I'm not surprised because the datasheet makes no mention of doing this. Oh well, I thought it was worth a shot.
If you MUST use the 34063 you could replace the 0.22 ohm resistor with a wire link, then use the output of the 34063 to drive a logic level NFET with a low Rds. After all that you might get the efficiency to 70% and get 5.7v 80mA out from about 3v 150mA input.
Can you elaborate on this idea please?
Why do you need 80mA? High efficiency LEDs will light up from 1mA each, the new PICs will run from maybe 2mA if you don't use a high freq xtal, so how much do the inclinometer IC and Xbee require?
The 80 mA requirement comes from:
Xbee = 50 mA (Peak RX current)
PIC = 1.8 mA (Internal 4 MHz oscillator)
Inclinometer = 4 mA (Maximum current consumption @ 5V V
DD from the datasheet)
2 x LED = 2*10 = 20 mA
The LED is not high efficiency (although in hindsight it probably should have been), it is a bi-color red/green LED. I sized their resistors so they are only drawing 10 mA each.
All that adds up to 75.8 mA. I just upped it to 80 mA to be safe.
And why 5.7v? Were you going to add a low dropout linear 5v post regulator?
The 5.7V will supply a 5V LDO linear regulator which will supply the PIC, inclinometer IC, and LEDs. I chose the
LP2985-50DBVT. The max dropout voltage will probably only be about 150 mV. I'm struggling to remember why I decided on the 5.7V. Maybe I just wanted a buffer but that still seems like a bit much.
What i noticed too is that you seem to be using a 1.5nf cap for the timing cap. What made you choose that over say 1nf? 1nf offers a little higher frequency which might be better because that means it works better overall in most cases.
I am using the 1.5nF cap because that was what was used in the datasheet for the example step up converter circuit. I did try a 1nF cap this weekend but unfortunately, it didn't work. Not sure why.
At 3v Vcc it will be worse, and worse still as the drive comes AFTER the 0.22 ohm current sense resistor which drops Vcc, and much worse again becuse the OP is using a 180ohm DRI resistor.
Similarly to the 1.5nF cap, I chose 180Ω because that was what was shown in the datasheet for the step up converter example circuit. I suppose that is one of the pit falls of my paint by numbers design methodology. I am not an EE and don't always understand how all the individual pieces work but I am usually pretty good at following directions and using them as building blocks. Usually being the operative word here.
If you're aginst using proprietary chips (national semi, texas instruments, linear tech, zetex etc..) then, for <100mA output, and not tight frequency requirements, you might find a discrete boost converter to be more efficient. They're generally much less versatile, limited input voltage range etc. but for a one-trick pony they can work surprisingly well
I'm not against using any proprietary chips. I just had a bunch of 34063s and inappropriately chose it for this project. Now I'm just trying to avoid starting over. For future projects though, I'll definitely consider the alternatives that you and others have identified for me.
So, the consensus so far seems to be that I should change the 180Ω resistor to 47Ω one and that I should replace the .22Ω resistor with a wire link. Unless anyone has any objections, I'll try that this afternoon.
Also, I did order some 3.7V AAA Li-ion batteries from China. If all else fails, I'll just bypass the 34063 on my PCB completely and power the two LDO linear regulators directly from the batteries. I think I'll keep trying to kick this dead horse until the batteries arrive though.