I needed a reasonably stable 6.2V power supply for my audio amplifier project. I've found a few AIC1563PN dc-dc converters (looks identical to MC34063 according to the datasheet) which I salvaged from some old modems and used one of them as a step-down converter.
I've connected it according to the datasheet with an extra LC-filter at the output to reduce rippling. The diode I've used is 1N5817. The circuit is powered from a non-regulated wall wart "9V 500mA" adapter that outputs around 14.5V with no load and drops to 14V when I connect it to the circuit with load. After measuring output voltage I had some strange results:
- Vout = 4.45V with no load
- Vout = 3.79V with 10k load
- Vout = 2.38V with 620 Ohm load
Why is it performing so bad? According to my calculations, Vout is supposed to be around 6.2V and since the converter is capable of delivering relatively high current, there shouldn't be such a huge voltage drop. Or should I use a different (better-performing) converter for this task?
Update: I've just measured the resistance of R2 and.. it was only a few ohms. However the color code is "orange-white-red-gold". Weird. I've checked all the components, but I never thought that something like a 3.9k resistor could malfunction. Wasted many hours because of that resistor 🙂 Anyway, thank you everyone for your effort and patience, it was a silly mistake after all.
Best Answer
MC34063 should work well as shown.
Internal switch is not marvellous but good enough in this and many other applications.
I'm wary of the AIC1563 - is pin 8 identical to 34063 - may be but be sure.
Check all connections are as you think.
Use an ohmmeter to ensure adjacent pins and tracks are not shorted.
Use ohmmeter on top of perf board to be sure things are connected as expected.
Ensure timing capacitor is REALLY 330 pF and not something larger.
Short Isense resistor - but you say you did that.
Try small cap across R2 - upper R of voltage divider. 1 NF or less.
Swap IC for another - but probably not the problem.