MC33363A
www.onsemi.com
9
OPERATING DESCRIPTION
Introduction
The MC33363A represents a new higher level of
integration by providing all the active high voltage power,
control, and protection circuitry required for
implementation of a flyback or forward converter on a single
monolithic chip. This device is designed for direct operation
from a rectified 240 Vac line source and requires a minimum
number of external components to implement a complete
converter. A description of each of the functional blocks is
given below, and the representative block and timing
diagrams are shown in Figures 19 and 20.
Oscillator and Current Mirror
The oscillator frequency is controlled by the values
selected for the timing components R
T
and C
T
. Resistor R
T
programs the oscillator charge/discharge current via the
Current Mirror 4 I output, Figure 4. Capacitor C
T
is charged
and discharged by an equal magnitude internal current
source and sink. This generates a symmetrical 50%
duty cycle waveform at Pin 7, with a peak and valley
threshold of 2.6 V and 0.6 V respectively. During the
discharge of C
T
, the oscillator generates an internal blanking
pulse that holds the inverting input of the AND gate
driver high. This causes the Power Switch gate drive to be
held in a low state, thus producing a well controlled amount
of output deadtime. The amount of deadtime is relatively
constant with respect to the oscillator frequency when
operating below 1.0 MHz. The maximum Power Switch
duty cycle at Pin 16 can be modified from the internal 50%
limit by providing an additional charge or discharge current
path to C
T
, Figure 21. In order to increase the maximum duty
cycle, a discharge current resistor R
D
is connected from
Pin 7 to ground. To decrease the maximum duty cycle, a
charge current resistor R
C
is connected from Pin 7 to the
Regulator Output. Figure 5 shows an obtainable range of
maximum output duty cycle versus the ratio of either R
C
or
R
D
with respect to R
T
.
Figure 21. Maximum Duty Cycle Modification
PWM
Current
Regulator Output
1.0
R
C
8
6
2.25 I
I
T
T
Mirror
4 I
Oscillator
Comparator
R
D
R
C
7
Current
Limit
Reference
Blanking
Pulse
The formula for the charge/discharge current along with
the oscillator frequency are given below. The frequency
formula is a first order approximation and is accurate for C
T
values greater than 500 pF. For smaller values of C
T
, refer to
Figure 2. Note that resistor R
T
also programs the Current
Limit Comparator threshold.
I
chgńdscg
+
5.4
R
T
f [
I
chgńdscg
4C
T
PWM Comparator and Latch
The pulse width modulator consists of a comparator with
the oscillator ramp voltage applied to the non−inverting
input, while the error amplifier output is applied into the
inverting input. The Oscillator applies a set pulse to the
PWM Latch while C
T
is discharging, and upon reaching the
valley voltage, Power Switch conduction is initiated. When
C
T
charges to a voltage that exceeds the error amplifier
output, the PWM Latch is reset, thus terminating Power
Switch conduction for the duration of the oscillator ramp−up
period. This PWM Comparator/Latch combination
prevents multiple output pulses during a given oscillator
clock cycle. The timing diagram shown in Figure 20
illustrates the Power Switch duty cycle behavior versus the
Compensation voltage.
Current Limit Comparator and Power Switch
The MC33363A uses cycle−by−cycle current limiting as
a means of protecting the output switch transistor from
overstress. Each on−cycle is treated as a separate situation.
Current limiting is implemented by monitoring the output
switch current buildup during conduction, and upon sensing
an overcurrent condition, immediately turning off the switch
for the duration of the oscillator ramp−up period.
The Power Switch is constructed as a SENSEFET
allowing a virtually lossless method of monitoring the drain
current. It consists of a total of 2819 cells, of which 65 are
connected to a 6.0 W ground−referenced sense resistor. The
Current Sense Comparator detects if the voltage across the
sense resistor exceeds the reference level that is present at
the inverting input. If exceeded, the comparator quickly
resets the PWM Latch, thus protecting the Power Switch.
The current limit reference level is generated by the 2.25 I
output of the Current Mirror. This current causes a reference
voltage to appear across the 450 W resistor. This voltage
level, as well as the Oscillator charge/discharge current are
both set by resistor R
T
. Therefore when selecting the values
for R
T
and C
T
, R
T
must be chosen first to set the Power
Switch peak drain current, while C
T
is chosen second to set
the desired Oscillator frequency. A graph of the Power
Switch peak drain current versus R
T
is shown in Figure 3
with the related formula below.
I
pk
+ 15.95
ǒ
R
T
1000
Ǔ
− 1.14
The Power Switch is designed to directly drive the
converter transformer and is capable of switching a
