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MC33067PG

(MC33067 / MC34067) HIGH PERFORMANCE ZERO VOLTAGE SWITCH RESONANT MODE CONTROLLERS

ON Semiconductor 

MC34067, MC33067

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 and 15 ns

respectfully 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.

VCC

15

Enable /

UVLO Adjust

9

50k

7.0k

50k

7.0k

8.0 V VCC UVLO

5.1 V

Reference

Vref

Vref UVLO

Vref

5

UVLO

Figure 19. Undervoltage Lockout and Reference

4.2/4.0 V

PERIPHERAL SUPPORT FUNCTIONS

The MC34067 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 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

input VCC voltage and the regulated reference voltage as

illustrated in Figure 19. When VCC increases to the upper

threshold voltage, the VCC UVLO comparator enables the

Reference Regulator. After the Vref output of the Reference

Regulator rises to 4.2 V, the Vref UVLO comparator switches

the UVLO signal to a logic zero state enabling the primary

control path. Reducing VCC to the lower threshold voltage

causes the VCC UVLO comparator to disable the Reference

Regulator. The Vref UVLO comparator then switches the

UVLO output to a logic one state disabling the controller.

The Enable/UVLO Adjust pin allows the power supply

designer to select the VCC UVLO threshold voltages. When

this pin is open, the comparator switches the controller on at

16 V and off at 9.0 V. If this pin is connected to the VCC

terminal, the upper and lower thresholds are reduced to

9.0 V and 8.6 V, respectively. Forcing the Enable/UVLO

Adjust pin low will pull the VCC UVLO comparator input

low (through 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 Detection

Converter protection from adverse operating conditions

can be implemented with proper use of the Fault Comparator

and Latch blocks that are illustrated in Figure 20. The Fault

Comparator has an input threshold of 1.0 V and when

exceeded, sets the Fault Latch and generates two logic

signals that simultaneously disable the primary control path.

The signal line labeled “Fault” connects directly to two gates

that control the output drivers. This direct path reduces the

driver turn−off propagation delay to approximately 70 ns.

The Fault Latch output is OR’ed with the UVLO output that

is derived from the Vref UVLO comparator, to produce the

logic output labeled “UVLO+Fault”. This signal disables

the Oscillator and the One−Shot by forcing both the COSC

and CT capacitors to be continually charged.

The Fault Latch is automatically reset during startup by a

logic “1” that appears at the Vref UVLO comparator output.

The latch can also be reset after startup by momentarily

pulling the Enable/UVLO Adjust pin low to disable the

Reference. Note that after activation, the Fault Latch will

remain in a set state only as long as VCC is provided to the

MC34067. Also, Drive Output B will assume a high state if

the Fault input signal drops below the 1.0 V threshold level

even after the Fault Latch has been set. In some applications

this characteristic could be problematic but it can be easily

remedied by AC coupling Drive Output B.

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