MC34066 Datasheet, PDF (8/12 Pages) ON Semiconductor – High Performance High Performance

VCC
15
Enable/
UVLO Adjust ₉
MC34066 MC33066
Figure 6. Undervoltage Lockout and Reference
50k
7k
7k
+
Reference
50k
–
Regulator
VCC UVLO
8.0V
5.1V Vref UVLO _–
UVLO
+
4.2V/4.0V
5 ^Vref
4 ^Gnd
The Enable/UVLO Adjust terminal 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 Detector
The high–speed Fault Comparator and Latch illustrated in
Figure 7 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 Latch output is
OR’d with UVLO output from the Vref UVLO comparator to
produce the logic output labeled UVLO + Fault. This signal
disables the Oscillator and One–Shot by forcing both the
COSC and CT capacitors to be continually charged.
Figure 7. Fault Detector and Soft–Start
UVLO + Fault UVLO
Fault
EA Clamp
9µA
Soft–Start
Buffer
R
QS
Fault
Latch
+
Fault ^–
Comparator _1V
Fault
Input
10
CSoft–
Start
11
The Fault Latch is reset during startup by a logic one at the
UVLO output of the Vref UVLO comparator. The latch can 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 7 forces the variable
frequency Oscillator to start at the minimum frequency and
ramp upward until regulated by the feedback control loop.
The external capacitor at the CSoft–Start 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 be disabled by simply opening the
CSoft–Start terminal.
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.
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