UC3842B, UC3843B, UC2842B, UC2843B, NCV3843BV
V
CC
7(12)
V
in
5.0V Ref
8(14)
5.0V Ref
8(14)
R
Bias
Osc
R
+
−
Osc
4(7)
+
1.0mA
2(3)
1.0M
1(1)
C
t
Soft−Start
≈
3600C in
mF
5(9)
R
Bias
R
+
−
+
−
7(11)
Q1
6(10)
S
Q
R
1.0V
Comp/Latch
3(5)
5(9)
4(7)
+
1.0 mA
V
Clamp
2(3)
S
Q
2R
R
R
1.0V
C
R
2
1(1)
R
1
MPSA63
EA
2R
R
5(8)
R
S
EA
VClamp
[
1.67
R2
)
1
R1
Where: 0
≤
V
Clamp
≤
1.0 V
VC
R1 R2
C
tSoft-Start
+ *
In 1
*
R1
)
R2
3 VClamp
Ipk(max)
[
VClamp
RS
Figure 24. Soft−Start Circuit
Figure 25. Adjustable Buffered Reduction of
Clamp Level with Soft−Start
V
CC
(12)
V
in
VPin 5
[
RS Ipk rDS(on)
rDM(on)
)
RS
V
CC
7(12)
V
in
If: SENSEFET = MTP10N10M
R
S
= 200
5.0V Ref
+
−
D
+
−
(11)
G
K
(10)
S
Q
R
Comp/Latch
(5)
Control Circuitry Ground:
To Pin (9)
Virtually lossless current sensing can be achieved with the implementation of a
SENSEFET power switch. For proper operation during over−current conditions, a
reduction of the I
pk(max)
clamp level must be implemented. Refer to Figures 23 and 25.
R
S
1/4 W
Power Ground:
To Input Source
Return
(8)
R
Comp/Latch
S
Q
M
Then : VPin 5
[
0.075 Ipk
5.0V Ref
SENSEFET
S
+
−
+
−
7(11)
Q1
6(10)
5(8)
3(5)
C
R
R
S
The addition of the RC filter will eliminate instability caused by the leading
edge spike on the current waveform.
Figure 26. Current Sensing Power MOSFET
Figure 27. Current Waveform Spike Suppression
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