MC34025, MC33025
http://onsemi.com
14
V
ref
R
2
R
1
C
SS
1
2
8
+
Figure 32. Buffered Maximum Clamp Level
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
1
.
The new equation for Soft−Start is
t [
V
clamp
) 0.6
9.0 μA
ǒ
C
SS
Ǔ
In current mode operation, this circuit will limit the maximum
voltage allowed at the ramp input to end a cycle.
Q
Q
T
V
in
To Current
Sense Input
R
S
12
11
14
15
V
C
Base Charge
Removal
0
-
+
I
B
Figure 33. Bipolar Transistor Drive
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.
Q
Q
T
12
11
14
15
V
C
Isolation
Boundary
Figure 34. Isolated MOSFET Drive
12
11
14
15
Q
Q
T
V
C
V
C
Figure 35. Direct Transformer Drive
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.
Q
Q
T
V
in
To Current
Sense Input
R
S
12
11
14
15
V
C
Figure 36. MOSFET Parasitic Oscillations
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.
