How 555 pwm works
D1 and D2 steer the timing capacitor current through R1 while the output is high and R2 while it is low. Keeping R1 + R2 constant approximately preserves frequency while adjusting duty. This output-feedback arrangement makes either side of 50% duty available.
Ideal steering diodes and one-third/two-thirds supply thresholds. The mapped behavioral LM555 has rail-to-rail output; real timer output drop and diode forward voltage change frequency and duty.
Change the form inputs or click a component value on the schematic. Choose Try in CircuitSim to simulate the configured circuit.