UC3844, UC3845, UC2844, UC2845
V
CC
7(12)
+
−
V
in
+
0
+
−
7(11)
R
g
T
−
+
Q
R
Comp/Latch
S
6(10)
5(8)
3(5)
R
S
Series gate resistor R
g
will damp any high frequency parasitic oscillations
caused by the MOSFET input capacitance and any series wiring inductance
in the gate−source circuit.
Q1
Q1
6(1)
5(8)
3(5)
R
S
The totem−pole output can furnish negative base current for enhanced
transistor turn−off, with the addition of capacitor C
1
.
−
Base
Charge
Removal
I
B
V
in
5.0V
ref
+
−
+
−
C
1
Figure 25. MOSFET Parasitic Oscillations
Figure 26. Bipolar Transistor Drive
8(14)
R
Bias
R
V
CC
7(12)
+
−
Isolation
Boundary
V
in
4(7)
+
−
2(3)
V
GS
Waveforms
+
0
−
+
OSC
1.0mA
EA
2R
R
5.0V
ref
+
−
+
−
7(11)
−
T
S
Q
−
R
+
Comp/Latch
6(10)
5(8)
R
3(5) C
R
S
N
S
I
pk
=
N
p
Figure 27. Isolated MOSFET Drive
From V
O
2.5V
R
i
R
d
C
I
2(3)
R
f
1(1)
R
f
≥
8.8 k
5(9)
+
−
EA
+
1.0mA
2R
R
Error Amp compensation circuit for stabilizing any current−mode topology except
for boost and flyback converters operating with continuous inductor current.
Figure 29. Error Amplifier Compensation
ÉÉ É É
É É
É ÉÉ
50% DC
+
0
−
25% DC
N
P
N
S
V
(pin 1)
− 1.4
3 R
S
+
Q1
1(1)
MCR
101
2N
3905
2N
3903
5(9)
The MCR101 SCR must be selected for a holding of less than 0.5 mA at T
A(min)
.
The simple two transistor circuit can be used in place of the SCR as shown. All
resistors are 10 k.
Figure 28. Latched Shutdown
From V
O
R
p
C
p
R
i
R
d
C
I
2(3)
R
f
1(1)
2.5V
+
−
+
1.0mA
EA
2R
R
5(9)
Error Amp compensation circuit for stabilizing current−mode boost and flyback
topologies operating with continuous inductor current.
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