Why does MC34063 switch behave so?

[beowulfenator]

Hi!

I'm trying to understand how the switch inside the MC34063 works. I've put together a test circuit.

I thought I'd see a rectangular waveform on pin 1, the switch being open for 6t and pulling pin 1 low to ground, then the switch being closed for 1t and letting pin 1 go high to supply voltage trough the resistor. Here's what I measure, though.

The probes are 10x, so vertically one cell = 1V. The upper trace is from the timing capacitor (pin 3). The lower trace (on pin 1) I don't quite understand.

When the switch is open, pin 1 is slightly above ground (PN junction voltage?). But then when the switch closes, the trace abruptly falls below 0, and then it slowly climbs up but not anywhere near the supply voltage. I can also see that it's "accelerating", the increase begins slow and then goes faster and faster.

Can anyone tell me what is going on here?

[mikerj]

That 27k pull-up is way to much resistance, don't forget that transistor would be switching up at 1.5 Amp in it's intended use, so you may be seeing leakage effects.

Also use a separate resistor to supply the driver transistor on pin 8 from Vcc, as it's wired you've configured like a Darlington which will not be able to pull down very close to 0v.

[beowulfenator]

Thank you!

I've increased the load by changing the resistor to 1K, and the numbers are now exactly as in the datasheet (down to roughly 1V and up to almost Vcc).