MC34066 Datasheet, PDF (5/12 Pages) ON Semiconductor – High Performance High Performance

MC34066 MC33066
Oscillator
The characteristics of the variable frequency Oscillator are
crucial for precise controller performance at high operating
frequencies. In addition to triggering the One–Shot timer and
initiating the output pulse, the Oscillator also determines the
initial voltage for the One–Shot capacitor and defines the
minimum deadtime between output pulses. The Oscillator is
designed to operate at frequencies exceeding 1.0 MHz. The
Error Amplifier can control the oscillator frequency over a
1000:1 frequency range, and both the minimum and
maximum frequencies are easily and accurately
programmed by the proper selection of external components.
The Oscillator also includes an adjustable deadtime feature
for applications requiring additional time between output
pulses.
The functional diagram of the Oscillator and One–Shot
timer is shown in Figure 2. The oscillator capacitor COSC is
initially charged by transistor Q1 through the optional
deadtime resistor RDT. When COSC exceeds the 4.9 V upper
threshold of the oscillator comparator, the base of Q1 is
pulled low allowing COSC to discharge through the external
resistors and the internal Current Mirror. When the voltage on
COSC falls below the comparator’s 3.6 V lower threshold, Q1
turns on and again charges COSC.
2.5ROSC _{+ 5.1}
tdchg(min) = (RDT + ROSC)COSCIn ^RVFO
(1)
2.5ROSC _{+ 3.6}
RVFO
The minimum oscillator frequency will result when the IOSC
current is zero, and COSC is discharged through the external
resistors ROSC and RDT. This occurs when the Error Amplifier
output voltage is less than the two diode drops required to
bias the input of the Current Mirror. The maximum oscillator
discharge time is given by Equation 2.
tdchg(max) = (RDT + ROSC) COSCIn
5.1
3.6
(2)
The outputs of the control IC are off whenever the oscillator
capacitor COSC is being charged by transistor Q1. The
minimum time between output pulses (deadtime) can be
programmed by controlling the charge time of COSC. Resistor
RDT reduces the current delivered by Q1 to COSC, thus
increasing the charge time and output deadtime. Varying RDT
from 0 Ω to 1000 Ω will increase the output deadtime from
80 ns to 680 ns with COSC equal to 300 pF. The general
expression for the oscillator charge time is give by
Equation 3.
Figure 2. Oscillator and One–Shot Timer
VCC
tchg(max) = RDT COSC In
5.1–3.6 _{+ 80 ns}
5.1–4.9
(3)
Osc Deadtime
ROSC
1
RDT
Osc RC
2
COSC
One–Shot RC
RT
16
CT
Osc Control
Current
3
IOSC
RVFO
6
Error Amp
Output
Q1
IOSC
Q2
5.1V
Oscillator
–
+
4.9V/3.6V
One–Shot
–
+
4.9V/3.6V
UVLO + Fault
Current Mirror
If RDT is 0 Ω, COSC charges from 3.6 V to 5.1 V in less than
50 ns. The high slew rate of COSC and the propagation delay
of the comparator make it difficult to control the peak voltage.
This accuracy issue is overcome by clamping the base of Q1
through diode Q2 to a voltage reference. The peak voltage of
the oscillator waveform is thereby precisely set at 5.1 V.
The frequency of the Oscillator is modulated by varying the
current IOSC flowing through RVFO into the Osc Control
Current pin. The control current drives a unity gain Current
Mirror which pulls an identical current from the COSC
capacitor. As IOSC increases, COSC discharges faster thus
decreasing the Oscillator period and increasing the
frequency. The maximum frequency occurs when the Error
Amplifier output is at the upper clamp level, nominally 2.5 V
above the voltage at the Osc Control Current pin. The
minimum discharge time for COSC, which corresponds to the
maximum oscillator frequency, is given by Equation 1.
The minimum and maximum oscillator frequencies are
programmed by the proper selection of resistor ROSC and
RVFO. After selecting RDT for the desired deadtime, the
minimum frequency is programmed by ROSC using
Equations 2 and 3 in Equation 4:
1=
fOSC(min)
tdchg(max)
+
tchg
(4)
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 2.
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)
MOTOROLA ANALOG IC DEVICE DATA
5