UC3842B, UC3843B, UC2842B, UC2843B
http://onsemi.com
13
+
-
+
-
S
R
+
R
R
R
2R
V
Clamp
[
1.67
ǒ
R
2
R
1
) 1
Ǔ
I
pk(max)
[
V
Clamp
R
S
5.0V Ref
Q
Bias
Osc
EA
1.0V
5(9)
7(11)
6(10)
5(8)
3(5)
R
S
Q1
V
CC
V
in
1(1)
2(3)
4(7)
8(14)
R
1
V
Clamp
R
2
Where: 0 ≤ V
Clamp
≤ 1.0 V
C
MPSA63
t
Soft
Start
+*In
ƪ
1 *
V
C
3V
Clamp
ƫ
C
R
1
R
2
R
1
)R
2
7(12)
1.0 mA
Comp/Latch
5.0V Ref
+
-
S
R
Q
Bias
+
1.0mA
Osc
R
R
R
2R
EA
1.0V
5(9)
1(1)
2(3)
4(7)
8(14)
C
1.0M
t
Soft-Start
≈ 3600C in mF
Figure 25. Soft−Start Circuit Figure 26. Adjustable Buffered Reduction of
Clamp Level with Soft−Start
+
-
5.0V Ref
+
-
S
R
Q
(11)
(10)
(8)
Comp/Latch
(5)
R
S
1/4 W
V
CC
V
in
K
M
D
SENSEFET
G
S
Power Ground:
To Input Source
Return
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 24 and 26.
V
Pin 5
[
R
S
I
pk
r
DS(on)
r
DM(on)
) R
S
If: SENSEFET = MTP10N10M
R
S
= 200
Then : V
Pin5
[ 0.075I
pk
(12)
Figure 27. Current Sensing Power MOSFET
+
-
5.0V Ref
+
-
S
R
Q
7(11)
6(10)
5(8)
3(5)
R
S
Q1
V
CC
V
in
C
R
The addition of the RC filter will eliminate instability caused by the leading
edge spike on the current waveform.
7(12)
Comp/Latch
Figure 28. Current Waveform Spike Suppression
