MC33066 View Datasheet(PDF) - ON Semiconductor

Part Name

Description

Manufacturer

MC33066

High Performance Resonant Mode Controllers

ON Semiconductor 

MC34066, MC33066

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 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 3. 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.

VCC

Osc Deadtime

Q1

ROSC

1

RDT

Osc RC

2

COSC

IOSC

Q2

5.1V

Oscillator

−

+

4.9V/3.6V

One−Shot RC

RT

16

CT

Osc Control

Current

One−Shot

−

+

4.9V/3.6V

3

UVLO + Fault

IOSC

RVFO

6

Current Mirror

Error Amp

Output

Figure 3. Oscillator and One−Shot Timer

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.

tdchg(min) = (RDT + ROSC)COSCIn

2.5ROSC

RVFO

+ 5.1

(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.

tchg(max) = RDT COSC In

5.1−3.6

5.1−4.9

+ 80 ns

(3)

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)

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