MC34023 ON Semiconductor, MC34023 Datasheet - Page 14

• Manufacturer Part Number: MC34023
• Description: (MC33023 / MC34023) High Speed Single-Ended PWM Controller
• Manufacturer: ON Semiconductor

In voltage mode operation, the maximum duty cycle can be clamped. By
the addition of a PNP transistor to buffer the clamp voltage, the Soft−Start
current is not affected by R
In current mode operation, this circuit will limit the maximum voltage
allowed at the ramp input to end a cycle.
A series gate resistor may be needed to dampen high frequency parasitic
oscillation caused by the MOSFET’s input capacitance and any series
wiring inductance in the gate−source circuit. The series resistor will also
decrease the MOSFET switching speed. A Schottky diode can reduce
the driver’s power dissipation due to excessive ringing, by preventing the
output pin from being driven below ground. The Schottky diode also
prevents substrate injection when the output pin is driven below ground.
The new equation for Soft−Start is
Figure 31. Buffered Maximum Clamp Level
Figure 33. MOSFET Parasitic Oscillations
R
R
1
2
V
ref
1
C
.
SS
15
14
12
1
2
8
V
C
t [
V clamp ) 0.6
Sense Input
To Current
9.0 μA
+
MC34023, MC33023
V
in
(
C
R
http://onsemi.com
SS
S
)
14
+
0
−
I
The totem pole output can furnish negative base current for enhanced
transistor turn−off, with the addition of the capacitor in series with the base.
The totem pole output can easily drive pulse transformers. A Schottky
diode is recommended when driving inductive loads at high frequencies.
The diode can reduce the driver’s power dissipation due to excessive
ringing, by preventing the output pin from being driven below ground.
B
Figure 34. Isolated MOSFET Drive
Figure 32. Bipolar Transistor Drive
Base Charge
Removal
15
14
12
V
C
15
14
12
V
C
Sense Input
To Current
V
in
R
S