MC34067, MC33067
The totem−pole output drivers are ideally suited for driving
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.
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.
V
CC
15
7.0k
50k
50k
7.0k
V
ref
Enable /
UVLO Adjust
5.1 V
Reference
V
ref
9
5
8.0 V
V
ref
UVLO
V
UVLO
CC
4.2/4.0 V
UVLO
Figure 19. Undervoltage Lockout and Reference
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.
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
Undervoltage Lockout and Voltage Reference
Separate undervoltage lockout comparators sense the
input V voltage and the regulated reference voltage as
CC
illustrated in Figure 19. When V increases to the upper
CC
threshold voltage, the V UVLO comparator enables the
CC
Reference Regulator. After the V output of the Reference
ref
Regulator rises to 4.2 V, the V UVLO comparator switches
is derived from the V UVLO comparator, to produce the
ref
ref
the UVLO signal to a logic zero state enabling the primary
logic output labeled “UVLO+Fault”. This signal disables
control path. Reducing V to the lower threshold voltage
the Oscillator and the One−Shot by forcing both the C
CC
OSC
causes the V UVLO comparator to disable the Reference
and C capacitors to be continually charged.
T
CC
Regulator. The V UVLO comparator then switches the
The Fault Latch is automatically reset during startup by a
ref
UVLO output to a logic one state disabling the controller.
The Enable/UVLO Adjust pin allows the power supply
logic “1” that appears at the V UVLO comparator output.
ref
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
designer to select the V UVLO threshold voltages. When
CC
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 V
remain in a set state only as long as V is provided to the
CC
CC
terminal, the upper and lower thresholds are reduced to
9.0 V and 8.6 V, respectively. Forcing the Enable/UVLO
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.
Adjust pin low will pull the V UVLO comparator input
CC
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
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