IR2153 frequency question

[Gavin]

I have the IR2153 configured for around 20KHZ by using a 3K3 and 10nf with no load conditions. Pin 6 is grounded to circuit ground ( -ve bridge) which will give me a square wave on both Hi and Lo sides. Ive bought a new digital scope which now allows me to see exactly whats going on in real time. What ive noticed is that the frequency on both outputs steadily increases by around 20hz every second for a few minutes then approx. 10hz every second. I can let this run for a while and see the frequency go from 20KHZ to 23KHZ in a short space of time.
I changed the 3K3 to a 5K pot so frequency can be adjusted and I get the same variation in frequency.
My understanding is that the frequency should be static with determined values.. so can anyone tell me why the increase?

Many thanks...

 

[KX36]

The oscillator of an IR2153 is a 555 timer although Rt is driven from what would be the square wave output in the 555. This is a good chance to play with your new scope. Is the peak voltage at the Rt pin changing over time? Anything else interesting you can see from the Ct and Rt waveforms? Logically, either that voltage is changing, or the current into the Ct pin is changing or the R and/or C values are drifting over time/temperature.