MC34066, MC33066
The maximum oscillator frequency is set by resistor RVFO
in a similar fashion using Equations 1 and 3 in Equation 5:
1
fOSC(max)
= tdchg(min) + tchg
(5)
The value chosen for resistor RDT will affect the peak
voltage of the oscillator waveform. As RDT is increased
from zero, the time required to charge COSC becomes large
with respect to the propagation delay through the oscillator
comparator. Consequently, the overshoot of the upper
threshold is reduced and the peak voltage on the oscillator
waveform drops from 5.1 V to 4.9 V. The best frequency
accuracy is achieved when RDT is zero ohms.
One−Shot Timer
The One−Shot capacitor CT is charged concurrently with
the oscillator capacitor by transistor Q1, as shown in
Figure 3. The One−Shot period begins when the oscillator
comparator turns off Q1, allowing CT to discharge. The
period ends when resistor RT discharges CT to the threshold
of the One−Shot comparator. Discharging CT from an initial
voltage of 5.1 V to a threshold voltage of 3.6 V results in the
One−Shot period given by Equation 6.
tOS = RT CT In
5.1
3.6
= 0.348 RT CT
(6)
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6