When the output is high, R3 and R4 are in parallel, and CI charges via Rl until the current in R2 equals that a
t the noninverting terminal. This action occurs when CI"s voltage rises to 2/3 of the supply voltage. At that point, the circuit switches regerier-atively. The output switches low and CI starts to discharge via Rl. Now, R4 is effectively disabled and the current to the noninverting terminal is determined solely by R3, so CI discharges until the current through R2 falls slightly below that of R3. This happens when the voltage across CI falls to about 1/3 of the supply voltage. At that point, the circuit again switches regeneratively, and the output again goes high. This circuit is useful for generating symmetrical square waves with maximum frequencies of only a few kHz. Because of the poor slew-rate characteristics of the LM3900 (0.5 V7/is), the output waveforms have rather slow rise and fall times.