The 566 can be wired as a positive or negative ramp generator. In the positive ramp generator, the external transistor driven by the Pin 3 output rapidly discharges Cl at the end of the charging period so that charging can resume instantaneously. The pnp transistor of the negative ramp generator likewise rapidly charges the timing capacitor Cl at the end of the discharge period. Because the circuits are reset so quickly, the temperature stability of the ramp generator is excellent. The period where f0 is the 566 free-running frequency in normal operation.
Therefore, where Vc is the bias voltage at Pin 5 and RT is the total resistance between Pin 6 and Vcc. Note that a short pulse is available at Pin 3. (Placing collector resistance in series with the external transistor collector will lengthen the pulse.)