MC33025 Motorola, MC33025 Datasheet - Page 14

• Manufacturer Part Number: MC33025
• Description: High Speed Double-Ended PWM Controller
• Manufacturer: Motorola

addition of a PNP transistor to buffer the clamp voltage, the Soft–Start current
is not affected by R 1 .
at the ramp input to end a cycle.
the driver’s power dissipation due to excessive ringing, by preventing the output pin from being driven below ground.
The new equation for Soft–Start is
In voltage mode operation, the maximum duty cycle can be clamped. By the
In current mode operation, this circuit will limit the maximum voltage allowed
14
T
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
T
Q
Q
Q
Q
Figure 31. Buffered Maximum Clamp Level
R 1
R 2
Figure 33. Isolated MOSFET Drive
V ref
C SS
1
2
8
t
[
15
14
11
12
15
14
11
12
V clamp
V C
V C
9.0
Figure 35. MOSFET Parasitic Oscillations
Boundary
)
Isolation
A
+
Sense Input
0.6
To Current
C
MC34025 MC33025
SS
V in
R S
transistor turn–off, with the addition of the capacitor in series with the base.
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
A series gate resistor may be needed to damp high frequency parasitic
I B
T
Q
Q
Figure 32. Bipolar Transistor Drive
T
Base Charge
Figure 34. Direct Transformer Drive
Removal
Q
Q
MOTOROLA ANALOG IC DEVICE DATA
15
14
11
12
V C
Sense Input
15
14
11
12
To Current
V C
V C
R S
V in