MC34066, MC33066
VCC
15
Enable/
UVLO Adjust 9
50k
7k
7k
+
50k
−
Reference
Regulator
VCC UVLO
8.0V
5.1V Vref UVLO −
UVLO
+
4.2V/4.0V
5 Vref
4 Gnd
Figure 7. Undervoltage Lockout and Reference
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 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
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.
UVLO + Fault UVLO
Fault
EA Clamp
9μA
Soft−Start
Buffer
R
QS
Fault
Latch
+
Fault −
Comparator 1V
Fault
Input
10
CSoft−
Start
11
Figure 8. Fault Detector and Soft−Start
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 8 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.
http://onsemi.com
9