I've been working on trying to get this SMPS to work for a while. I'm using an LTC1872, a step up DC controller with current limiting. The circuit used is the typical application provided in the datasheet:
With a couple of slight differences (since I'm not always able to get the correct parts):
- Capacitor at pin 2 is 200pF
- Output filter capacitors are a 100nF ceramic, 10uF electrolytic, and 2200uF electrolytic (scattergunning here a little)
- Inductor is ~3uH (calculated from an LC circuit)
The good news is that the converter does seem to work. I input 3V and was easily able to generate 6V (with no load) out of it:
However, when any sort of substantial load (<1kOhm), the output voltage rapidly drops - this is definitely undesirable since I need to be able to provide hundreds of milliamps to an amp for my project. When I used a 100R load, the waveform became this:
It appears to be pulsing towards target output voltage, but then abruptly terminating and thereafter switching off for the rest of the cycle (resulting in Vout = Vin). To me, this suggests that the current limiting is kicking in - the voltage is rising until the current gets too high, and then falls off as the converter stops boosting.
I've used a jumper cable for the sense resistor, but I have a feeling this has too high a resistance (compared to the one suggested). However, I've also tried shorting pins 5 and 4 with a piece of wire in between (the pins on the IC are less than a millimetre apart, so it's definitely going to be low resistance), and this doesn't seem to have too much of an effect. I'm not sure what else could be the problem. Perhaps the inductor saturating?
I've tried using a 45mOhm sense resistor (repurposed variable resistor) - doesn't seem to work. It's changing something, but I'm definitely not seeing an improvement. I've also tried using two inductors in series, which should give me something like 6uH. Also no luck.