MC33363A Motorola, MC33363A Datasheet - Page 9

• Manufacturer Part Number: MC33363A
• Description: HIGH VOLTAGE OFF-LINE SWITCHING REGULATOR
• Manufacturer: Motorola

The Power Switch is designed to directly drive the converter
transformer and is capable of switching a maximum of 700 V
and 1.0 A. Proper device voltage snubbing and heatsinking
are required for reliable operation.
sensing signal path. This circuit prevents a premature reset
of the PWM Latch. The premature reset is generated each
time the Power Switch is driven into conduction. It appears as
a narrow voltage spike across the current sense resistor, and
is due to the MOSFET gate to source capacitance,
transformer interwinding capacitance, and output rectifier
recovery time. The Leading Edge Blanking circuit has a
dynamic behavior in that it masks the current signal until the
Power Switch turn–on transition is completed. The current
limit propagation delay time is typically 300 ns. This time is
measured from when an overcurrent appears at the Power
Switch drain, to the beginning of turn–off.
Error Amplifier
inverting input and output is provided for primary side voltage
sensing, Figure 17. It features a typical dc voltage gain of 82
dB, and a unity gain bandwidth of 1.0 MHz with 78 degrees of
phase margin, Figure 5. The noninverting input is internally
biased at 2.6 V 3.1% and is not pinned out. The Error
Amplifier output is pinned out for external loop compensation
and as a means for directly driving the PWM Comparator.
The output was designed with a limited sink current capability
of 270 A, allowing it to be easily overridden with a pull–up
resistor. This is desirable in applications that require
secondary side voltage sensing, Figure 20. In this
application, the Voltage Feedback Input is connected to the
Regulator Output. This disables the Error Amplifier by placing
its output into the sink state, allowing the optocoupler
transistor to directly control the PWM Comparator.
Overvoltage Protection
eliminate the possibility of runaway output voltage. This
condition can occur if the control loop feedback signal path is
broken due to an external component or connection failure.
The comparator is normally used to monitor the primary side
V CC voltage. When the 2.6 V threshold is exceeded, it will
immediately turn off the Power Switch, and protect the load
from a severe overvoltage condition. This input can also be
driven from external circuitry to inhibit converter operation.
Undervoltage Lockout
incorporated to guarantee that the integrated circuit has
sufficient voltage to be fully functional before the output stage
is enabled. The UVLO comparator monitors the V CC voltage
at Pin 3 and when it exceeds 14.5 V, the reset signal is
removed from the PWM Latch allowing operation of the
Power Switch. To prevent erratic switching as the threshold is
crossed, 5.0 V of hysteresis is provided.
MOTOROLA ANALOG IC DEVICE DATA
An fully compensated Error Amplifier with access to the
An Overvoltage Protection Comparator is included to
An Undervoltage Lockout comparator has been
A Leading Edge Blanking circuit was placed in the current
MC33363A
Startup Control
enhancement mode MOSFET is included within the
MC33363A. This circuitry allows for increased converter
efficiency by eliminating the external startup resistor, and its
associated power dissipation, commonly used in most
off–line converters that utilize a UC3842 type of controller.
Rectified ac line voltage is applied to the Startup Input, Pin 1.
This causes the MOSFET to enhance and supply internal
bias as well as charge current to the V CC bypass capacitor
that connects from Pin 3 to ground. When V CC reaches the
UVLO upper threshold of 15.2 V, the IC commences
operation and the startup MOSFET is turned off. Operating
bias is now derived from the auxiliary transformer winding,
and all of the device power is efficiently converted down from
the rectified ac line.
20 mA, Figure 10, which decreases rapidly as V CC and the
die temperature rise. The steady state current will self limit in
the range of 8.0 mA with V CC shorted to ground. The startup
MOSFET is rated at a maximum of 400 V with V CC shorted to
ground, and 500 V when charging a V CC capacitor of
1000 F or less.
Regulator
biasing the Error Amplifier and any additional control system
circuitry. It is capable of up to 10 mA and has short–circuit
protection. This output requires an external bypass capacitor
of at least 1.0 F for stability.
Thermal Shutdown and Package
Switch in the event that the maximum junction temperature is
exceeded. When activated, typically at 155 C, the Latch is
forced into a ‘reset’ state, disabling the Power Switch. The
Latch is allowed to ‘set’ when the Power Switch temperature
falls below 145 C. This feature is provided to prevent
catastrophic failures from accidental device overheating. It is
not intended to be used as a substitute for proper
heatsinking.
dual–in–line package in which the die is mounted on a
special heat tab copper alloy lead frame. This tab consists of
the four center ground pins that are specifically designed to
improve thermal conduction from the die to the circuit board.
Figures 15 and 16 show a simple and effective method of
utilizing the printed circuit board medium as a heat dissipater
by soldering these pins to an adequate area of copper foil.
This permits the use of standard layout and mounting
practices while having the ability to halve the junction to air
thermal resistance. The examples are for a symmetrical
layout on a single–sided board with two ounce per square
foot of copper. Figure 22 shows a practical example of a
printed circuit board layout that utilizes the copper foil as a
heat dissipater. Note that a jumper was added to the layout
from Pins 8 to 10 in order to enhance the copper area near
the device for improved thermal conductivity. The application
circuit requires two ounce copper foil in order to obtain
8.0 watts of continuous output power at room temperature.
An internal Startup Control circuit with a high voltage
The startup MOSFET will provide an initial peak current of
A low current 6.5 V regulated output is available for
Internal thermal circuitry is provided to protect the Power
The MC33363A is contained in a heatsinkable plastic
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