MC34067, MC33067
http://onsemi.com
10
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.
Figure 19. Undervoltage Lockout and Reference
8.0 V
V
ref
15
9
V
CC
5
V
ref
4.2/4.0 V
V
ref
UVLO
V
CC
UVLO
7.0k
50k
7.0k
50k
5.1 V
Reference
Enable /
UVLO Adjust
UVLO
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 V
CC
voltage and the regulated reference voltage as
illustrated in Figure 19. When V
CC
increases to the upper
threshold voltage, the V
CC
UVLO comparator enables the
Reference Regulator. After the V
ref
output of the Reference
Regulator rises to 4.2 V, the V
ref
UVLO comparator switches
the UVLO signal to a logic zero state enabling the primary
control path. Reducing V
CC
to the lower threshold voltage
causes the V
CC
UVLO comparator to disable the Reference
Regulator. The V
ref
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 V
CC
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 V
CC
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 V
CC
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 V
ref
UVLO comparator, to produce the
logic output labeled “UVLO+Fault”. This signal disables
the Oscillator and the One
−
Shot by forcing both the C
OSC
and C
T
capacitors to be continually charged.
The Fault Latch is automatically reset during startup by a
logic “1” that appears at the V
ref
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 V
CC
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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