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
T is 1/2 fo
where f, is the 566 free-running frequency in normal operation. Therefore,
T = 1 = Rt C1 Vcc
2fo 5(Vcc - Vc)
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.) (Signetics, 1987 Linear Data Manual, Vol. 1: Communications, 2/87, p.
4-310) From figure FB80-1 of Encyclopedia of Electronic Circuits on CD-ROM Vol. 1.
