MC34066, MC33066

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9

Figure 7. Undervoltage Lockout and Reference

15

9

Enable/

UVLO Adjust

7k

50k

7k

50k

Reference

Regulator

5.1V

8.0V

UVLO

4.2V/4.0V

5

Gnd

4

−

+

−

+

The Enable/UVLO Adjust terminal allows the power

When this pin is open, the comparator switches the controller

terminal, the upper and lower thresholds are reduced to 9.0 V

and 8.6 V, respectively. Forcing the Enable/UVLO Adjust pin

an internal diode) turning off the controller.

The Reference Regulator provides a precise 5.1 V

reference to internal circuitry and can deliver up to 10 mA

to external loads. The reference is trimmed to better than 2%

initial accuracy and includes active short circuit protection.

Fault Detector

The high−speed Fault Comparator and Latch illustrated in

Figure 8 can protect a power supply from destruction under

fault conditions. The Fault Input pin connects to the input of

the Fault Comparator. If this input exceeds the 1.0 V

threshold of the comparator, the Fault Latch is set and two

logic signals simultaneously disable the primary control

path. The signal labeled Fault at the output of the Fault

Comparator is connected directly to the output drivers. This

direct path reduces the propagation delay from the Fault

Input to the A and B outputs to typically 70 ns. The Fault

UVLO comparator to produce the logic output labeled

UVLO + Fault. This signal disables the Oscillator and

continually charged.

Fault

Comparator

Q

R

S

Fault

Fault

Latch

1V

9

μA

10

Fault

Input

UVLO

UVLO + Fault

EA Clamp

Soft−Start

Buffer

Start

11

−

+

Figure 8. Fault Detector and Soft−Start

The Fault Latch is reset during startup by a logic one at the

also be reset after startup by pulling the Enable/UVLO Adjust

pin momentarily low to disable the Reference Regulator.

Soft−Start Circuit

The Soft−Start circuit shown in Figure 8 forces the

variable frequency Oscillator to start at the minimum

frequency and ramp upward until regulated by the feedback

terminal is initially discharged by the UVLO + Fault signal.

The low voltage on the capacitor pass through the Soft−Start

Buffer to hold the Error Amplifier output low. After UVLO

+ Fault switches to a logic zero, the soft−start capacitor is

charged by a 9.0 μA current source. The buffer allows the

Error Amplifier output to follow the soft−start capacitor

until it is regulated by the Error Amplifier inputs (or reaches

the 2.5 V clamp). The soft−start function is generally

applicable to controllers operating below resonance and can

APPLICATIONS

The MC34066 can be used for the control of series,

parallel or higher order half/full bridge resonant converters.

The IC is designed to provide control in discontinuous

conduction mode (DCM) or continuous conduction mode

(CCM) or a combination of the two. For example, in a

parallel resonant converter (PRC) operating in the DCM, the

IC is programmed to operate in fixed on−time, variable

frequency mode of operation. For a PRC operating in the

CCM, the IC can be programmed to operate in the variable

frequency mode with a fixed off−time.

When operating with a wide input voltage range, such as

a universal input power supply, a PRC can operate in the

DCM for high input voltage and in the CCM for low input

voltage. In this particular case, on−time is programmed

corresponding to DCM. The deadtime of the chip is

programmed to provide the desired off−time in the CCM.

The frequency range is chosen to cover the complete

frequency range from the DCM to the CCM. When

programmed as such, the controller will operate in the fixed

on−time, variable frequency mode at low frequencies. At the

frequency which causes the Oscillator to retrigger the

One−Shot, the control law changes to variable frequency

with fixed off−time. At higher frequencies the supply will

operate in the CCM with this control law.

Although the IC is designed and optimized for double ended

push−pull type converters, it can also be used for single ended

applications, such as forward and flyback resonant converters.