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The minimum frequency is programmed by R
Equation 1:
R
=
OSC
C
where t
is the internal propagation delay.
PD
The maximum oscillator frequency is set by the current
through resistor R
C
at the maximum oscillator frequency can be calculated
OSC
by Equation 2:
I
=
C OSC
(max)
The discharge current through R
and can be calculated by Equation 3:
I
R
OSC
Resistor R
VFO
R
One−Shot Timer
The One−Shot is designed to disable both outputs
simultaneously providing a deadtime before either output is
enabled. The One−Shot capacitor (C
concurrently with the oscillator capacitor by transistor Q1,
as shown in Figure 16. The one−shot period begins when the
oscillator comparator turns off Q1, allowing C
discharge. The period ends when resistor R
to the threshold of the One−Shot comparator. The lower
threshold of the One−Shot is 3.6 V. By choosing C
by solved by Equation 5:
R
T
Errors in the threshold voltage and propagation delays
through the output drivers will affect the One−Shot period.
To guarantee accuracy, the output pulse of the control chip
is trimmed to within 5% of 250 ns with nominal values of R
and C
.
T
The outputs of the Oscillator and One−Shot comparators
are OR'd together to produce the pulse t
Flip−Flop and output drivers. The output pulse (t
initiated by the Oscillator and terminated by the One−Shot
comparator. With zero voltage resonant mode converters,
the oscillator discharge time should never be set less than the
one−shot period.
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1
t
−
PD
t
−
ƒ
(max)
(min)
=
5.1
0.348
C OSC
n
OSC
3.6
. The current required to discharge
VFO
5.1 − 3.6
=
1.5C
OSC
1
ƒ
(max)
must also be known
OSC
1
ƒ
(min)
−
R OSC
C OSC
5.1 − 3.6
=
ε
R OSC
1
−
ƒ (min)
R OSC
C OSC
1.5
=
ε
R OSC
can now be calculated by Equation 4:
2.5 − V EAsat
=
VFO
−
I (max)
I R
OSC
T
T
t
t
OS
OS
=
=
5.1
n
0.348
C
T
3.6
, which drives the
OS
MC34067, MC33067
using
Error Amplifier
OSC
A fully accessible high performance Error Amplifier is
provided for feedback control of the power supply system.
The Error Amplifier is internally compensated and features
70 ns
dc open loop gain greater than 70 dB, input offset voltage of
(eq. 1)
less than 10 mV and a guaranteed minimum gain−bandwidth
product of 2.5 MHz. The input common mode range extends
from 1.5 V to 5.1 V, which includes the reference voltage.
ƒ (max)
I
OSC
(eq. 2)
Error Amp Output
Noninverting Input
Inverting Input
(eq. 3)
When the Error Amplifier output is coupled to the I
pin by R
Oscillator Control Current, I
Error Amplifier is restricted by a clamp circuit to improve
its transient recovery time.
(eq. 4)
Output Section
The pulse(t
timer is gated to dual totem−pole output drives by the
Steering Flip−Flop shown in Figure 18. Positive transitions
) is charged
of t
toggle the Flip−Flop, which causes the pulses to
OS
alternate between Output A and Output B. The flip−flop is
reset by the undervoltage lockout circuit during startup to
to
guarantee that the first pulse appears at Output A.
T
discharges C
T
, R
can
T
T
(eq. 5)
C T
T
) is
OS
Figure 18. Steering Flip−Flop and Output Drivers
http://onsemi.com
9
Oscillator
Control Current
3
R
VFO
6
8
7
Error
Amp
Figure 17. Error Amplifier and Clamp
, as illustrated in Figure 17, it provides the
VFO
. The output swing of the
OSC
), generated by the Oscillator and One−Shot
OS
V
Steering
Flip-Flop
PWR
Q
GND
T
V
Q
R
PWR
GND
3.1 V
Error Amp
Clamp
OSC
CC
Output A
14
Power Ground
13
CC
Output B
12
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