Other Parts Discussed in Thread: UC1843-SP, UC1843
I am planning to use the UC1843A-SP in a non-SMPS function where the frequency will be low, between 10Hz and 300Hz. Also, I need to stop the oscillator in the position where the output is low and, therefore, the RT/CT pin has to be forced to the Vref level of 5V for long periods of time. Actually, nearly all the time except for brief periods of activity. Questions, and preferably I'd need a more official answer from TI:
1. Can I use large capacitors if several uF? The datasheet stops at 100nF but that's just a graph.
2. Can I pull the RT/CT pin to VRef via a 10-30ohm resistance? This would pull the pin up against the 8.3mA discharge current, not much.
3. There is no abs max rating for the RT/CT pin. I assume Vref of 5V is ok. Correct?
4. What is the upper RT/CT pin voltage threshold where the oscillator flips into discharging the cap? The datasheet only states an amplitude of 1.7V but no upper or lower thresholds.
I know I can stop the chip via the current sense or comp pin but not in this case. The reason is that then the oscillator would keep running and will not be synchronized to our on-off signal. If I hold the RT/CT pin high the oscillator will always start in a defined state and thus the first pulse out of the chip will always be happening at the same delay. Given that I'd only be forcing against the 8.3mA discharge current I don't see any problem but since this is eventually going to fly I need an answer from TI about this.
Regards, Joerg