MC34066, MC33066

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8

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 ship

The outputs of the Oscillator and One−Shot comparators

initiated by the Oscillator, but either the oscillator

comparator or the One−Shot comparator can terminate the

pulse. When the oscillator discharge time exceeds the

one−shot period, the complete one−shot period is delivered

to the output section. If the oscillator discharge time is less

than the one−shot period, then the oscillator comparator

terminates the pulse prematurely and retriggers the

One−Shot. The waveforms on the left side of Figure 4

correspond to nonretriggered operation with constant

on−time and variable off−times. The right side of Figure 4

represents retriggered operation with variable on−time and

constant off−time.

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

less than 10 mV and 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.

For common mode voltages below 1.5 V, the Error

Amplifier output is forced low providing minimum

oscillator frequency.

The Oscillator Control Current pin is biased by the Error

Figure 5. The output swing of the Error Amplifier is

restricted by a clamp circuit to limit the maximum oscillator

accuracy is improved by trimming the clamp voltage to

value external components.

Osc Control

Current

3

Error Amp

Output

6

7

Error Amp

Inverting Input 8

Error Amp

Output Clamp

2.5V

EA Clamp

Error

Amplifier

−

+

−

+

Error Amp

Noninverting Input

Figure 5. Error Amplifier and Clamp

Output Section

timer is gated to dual totem pole output drives by the

Steering Flip−Flop shown in Figure 6. Positive transitions of

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.

The totem−pole output drives 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 MC34066 utilizes a

unique design that virtually eliminates cross conduction,

thus controlling the chip power dissipation at high

frequencies. A separate ground terminal is provided for the

output drivers to isolate the sensitive analog circuitry from

large transient currents.

Drivers

Steering

Flip−Flop

T

Q

Q

R

14

12

13

Drive

Output A

Drive

Output B

Drive

Gnd

Fault

UVLO

Figure 6. Steering Flip−Flop and Output Drivers

PERIPHERAL SUPPORT FUNCTIONS

The MC34066 Resonant Controller provides a number of

support and protection functions including a precision

voltage reference, undervoltage lockout comparators,

soft−start circuitry, and a fault detector. These peripheral

circuits ensure that the power supply can be turned on and

off in a safe, controlled manner and that the system will be

quickly disabled when a fault condition occurs.

Undervoltage Lockout and Voltage Reference

Separate undervoltage lockout comparators sense the

the UVLO signal to a logic zero state enabling the primary

UVLO output to a logic one state disabling the controller.