MC34067 Datasheet, PDF (7/16 Pages) ON Semiconductor – HIGH PERFORMANCE ZERO VOLTAGE SWITCH RESONANT MODE CONTROLLERS

MC34067 MC33067
OPERATING DESCRIPTION
Introduction
As power supply designers have strived to increase power
conversion efficiency and reduce passive component size,
high frequency resonant mode power converters have
emerged as attractive alternatives to conventional
pulse–width modulated control. When compared to
pulse–width modulated converters, resonant mode control
offers several benefits including lower switching losses,
higher efficiency, lower EMI emission, and smaller size. A
new integrated circuit has been developed to support this
trend in power supply design. The MC34067 Resonant Mode
Controller is a high performance bipolar IC dedicated to
variable frequency power control at frequencies exceeding
1.0 MHz. This integrated circuit provides the features and
performance specifically for zero voltage switching resonant
mode power supply applications.
The primary purpose of the control chip is to provide a
fixed off–time to the gates of external power MOSFETs at a
repetition rate regulated by a feedback control loop.
Additional features of the IC ensure that system startup and
fault conditions are administered in a safe, controlled manner.
A simplified block diagram of the IC is shown on the front
page, which identifies the main functional blocks and the
block–to–block interconnects. Figure 13 is a detailed
functional diagram which accurately represents the internal
circuitry. The various functions can be divided into two
sections. The first section includes the primary control path
which produces precise output pulses at the desired
frequency. Included in this section are a variable frequency
Oscillator, a One–Shot, a pulse Steering Flip–Flop, a pair of
power MOSFET Drivers, and a wide bandwidth Error
Amplifier. The second section provides several peripheral
support functions including a voltage reference, undervoltage
lockout, Soft–Start circuit, and a fault detector.
Primary Control Path
The output pulse width and repetition rate are regulated
through the interaction of the variable frequency Oscillator,
One–Shot timer and Error Amplifier. The Oscillator triggers
the One–Shot which generates a pulse that is alternately
steered to a pair of totem pole output drivers by a toggle
Flip–Flop. The Error Amplifier monitors the output of the
regulator and modulates the frequency of the Oscillator. High
speed Schottky logic is used throughout the primary
control channel to minimize delays and enhance high
frequency characteristics.
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 deadtime, the oscillator also determines
the initial voltage for the one–shot capacitor. 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 functional diagram of the Oscillator and One–Shot
timer is shown in Figure 14. The oscillator capacitor (COSC) is
initially charged by transistor Q1. 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 resistor, (ROSC), and the oscillator control current,
(IOSC). When the voltage on COSC falls below the
comparator’s 3.6 V lower threshold, Q1 turns on and again
charges COSC.
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 a diode to a voltage reference. The peak voltage of
the oscillator waveform is thereby precisely set at 5.1 V.
Figure 14. Oscillator and One–Shot Timer
+ ^VOE _Vref
+ _VOE
OSC Charge
ROSC
COSC
1
OSC RC
2
One–Shot RC
CT RT
Oscillator ¹⁰
Control Current
IOSO
3
RVFO
6
Error Amp Output
Q1
D1
IOSC
Oscillator
4.9V/3.6V
One–Shot
3.1V
Error Amp
Charge
4.9V/3.6V
The frequency of the Oscillator is modulated by varying
the current flowing out of the Oscillator Control Current (IOSC)
pin. The IOSC pin is the output of a voltage regulator. The
input of the voltage regulator is tied to the variable frequency
oscillator. The discharge current of the Oscillator increases
by increasing the current out of the IOSC pin. Resistor RVFO is
used in conjunction with the Error Amp output to change the
IOSC current. Maximum frequency occurs when the Error
Amplifier output is at its low state with a saturation voltage of
0.1 V at 1.0 mA.
The minimum oscillator frequency will result when the
IOSC current is zero, and COSC is discharged through the
external resistor (ROSC). This occurs when the Error
Amplifier output is at its high state of 2.5 V. The minimum and
maximum oscillator frequencies are programmed by the
proper selection of resistor ROSC and RVFO. The minimum
frequency is programmed by ROSC using Equation 1:
1–
ƒ_(min)
t_PD
t _(max)– 70 ns
ROSC
=
COSC
ȏn
ǒ
5.1
3.6
Ǔ
=
0.348 COSC
(1)
where tPD is the internal propagation delay.
MOTOROLA ANALOG IC DEVICE DATA
7