MC34067 Datasheet, PDF (8/16 Pages) ON Semiconductor – HIGH PERFORMANCE ZERO VOLTAGE SWITCH RESONANT MODE CONTROLLERS

MC34067 MC33067
The maximum oscillator frequency is set by the current
through resistor RVFO. The current required to discharge
COSC at the maximum oscillator frequency can be calculated
by Equation 2:
I(max) = COSC
5.1 – 3.6
1
ƒ_(max)
= 1.5COSC ƒ_{(max) (2)}
The discharge current through ROSC must also be known
and can be calculated by Equation 3:
1
ǒ Ǔ ƒ(min)
–
IROSC =
5.1 – 3.6
ROSC
ε
ROSCCOSC
(3)
ǒ–
1
Ǔ
= ^1.5 ε
ƒ_(min) ROSCCOSC
ROSC
Resistor RVFO can now be calculated by Equation 4:
RVFO
=
2.5 – VEAsat
I(max) – IROSC
(4)
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 (CT) is charged
concurrently with the oscillator capacitor by transistor Q1, as
shown in Figure 14. 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. The lower threshold
of the One–Shot is 3.6 V. By choosing CT, RT can by solved
by Equation 5:
RT
=
t _OS
CT
ȏn
ǒ
5.1
3.6
Ǔ
=
t _OS
0.348 CT
(5)
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 RT and
CT.
The outputs of the Oscillator and One–Shot comparators
are OR’d together to produce the pulse tOS, which drives the
Flip–Flop and output drivers. The output pulse (tOS) is
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.
Figure 15. Error Amplifier and Clamp
Oscillator
Control Current
IOSC
3
RVFO
6
Error Amp Output
8
Noninverting Input
Inverting Input ₇
Error
Amp
3.1V
Error Amp
Charge
When the Error Amplifier output is coupled to the IOSC pin
by RVFO, as illustrated in Figure 15, it provides the Oscillator
Control Current, IOSC. The output swing of the Error Amplifier
is restricted by a clamp circuit to improve its transient
recovery time.
Output Section
The pulse(tOS), generated by the Oscillator and One–Shot
timer is gated to dual totem–pole output drives by the
Steering Flip–Flop shown in Figure 16. Positive transitions of
tOS toggle the Flip–Flop, which causes the pulses to alternate
between Output A and Output B. The flip–flop is reset by the
undervoltage lockout circuit during startup to guarantee that
the first pulse appears at Output A.
Figure 16. Steering Flip–Flop and Output Drivers
VOE
±
Steering
Flip–Flop
Q
T
RQ
Pwr
Gnd
VOE
±
Output A
14
Power Ground
13
Output B
12
Pwr
Gnd
Error Amplifier
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 dc
open loop gain greater than 70 dB, input offset voltage of 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.
The totem–pole output drivers are ideally suited for driving
power MOSFETs and are capable of sourcing and sinking
1.5 A. Rise and fall times are typically 20 ns when driving a
1.0 nF load. High source/sink capability in a totem–pole
driver normally increases the risk of high cross conduction
current during output transitions. The MC34067 utilizes a
unique design that virtually eliminates cross conduction, thus
controlling the chip power dissipation at high frequencies. A
separate power ground pin is provided to isolate the sensitive
analog circuitry from large transient currents.
8
MOTOROLA ANALOG IC DEVICE DATA