MC34025 ON Semiconductor, MC34025 Datasheet - Page 14

• Manufacturer Part Number: MC34025
• Description: (MC33025 / MC34025) High Speed Single-Ended PWM Controller
• Manufacturer: ON Semiconductor

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.
In current mode operation, this circuit will limit the maximum
voltage allowed at the ramp input to end a cycle.
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
T
T
The new equation for Soft−Start is
Q
Q
Q
Q
Figure 32. Buffered Maximum Clamp Level
R
R
1
2
Figure 34. Isolated MOSFET Drive
V
ref
C
SS
1
2
8
t [
15
14
11
12
15
14
11
12
V
C
V clamp ) 0.6
V
C
Figure 36. MOSFET Parasitic Oscillations
9.0 μA
Boundary
Isolation
+
Sense Input
To Current
1
.
MC34025, MC33025
C
http://onsemi.com
SS
R
S
V
in
14
A series gate resistor may be needed to damp high frequency
parasitic oscillation caused by a MOSFET’s input capacitance
and any series wiring inductance in the gate−source circuit. The
series resistor will also decrease the MOSFET’s 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.
+
0
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.
−
T
I
B
Q
Q
Figure 33. Bipolar Transistor Drive
Base Charge
T
Removal
Figure 35. Direct Transformer Drive
Q
Q
15
14
11
12
V
C
Sense Input
To Current
15
14
11
12
V
V
C
C
R
V
S
in