UC3844, UC3845, UC2844, UC2845
V
CC
7(12)
+
−
V
in
5.0V
ref
8(14)
R
Bias
R
OSC
4(7)
R2
2(3)
1(1)
5(9)
V
Clamp
1.67
R
2
R
1
+1
+ 0.33 x 10
−3
R
1
R
2
R
1
+ R
2
+
−
+
1.0mA
EA
2R
R
−
+
1.0V
R1
+
−
V
Clamp
T
+
−
+
−
7(11)
Q1
6(10)
5(8)
4(7)
3(5)
1.0M
R
S
2(3)
1(1)
t
Soft−Start
3600C in
mF
5(9)
+
−
+
1.0mA
EA
2R
R
−
+
1.0V
C
8(14)
R
Bias
R
OSC
+
−
T
S
R
Q
+
−
5.0V
ref
Q
R
Comp/Latch
S
I
pk(max)
≈
V
Clamp
R
S
Where: 0
≤
V
Clamp
≤
1.0 V
Figure 20. Adjustable Reduction of Clamp Level
V
CC
7(12)
+
−
V
in
Figure 21. Soft−Start Circuit
5.0V
ref
8(14)
R
Bias
R
OSC
4(7)
+
−
2(3)
R2
1(1)
C
MPSA63
R1
V
Clamp
1.67
R
2
R
1
I
pk(max)
≈
V
Clamp
V
CC
+
−
7(11)
Q1
5.0V
ref
+
5(8)
3(5)
R
S
−
+
−
T
S
Q
R
Comp/Latch
(10) G
(8)
(5)
Control CIrcuitry
Ground:
To Pin (9)
M
+
−
+
−
+
−
(11)
(12)
V
in
V
Pin
5
≈
R
S
I
pk
r
DS(on)
r
DM(on)
+ R
S
+
−
V
Clamp
1.0mA
2R
R
+
−
+
T
−
+
S
Q
R
Comp/Latch
6(10)
If: SENSEFET = MTP10N10M
R
S
= 200
Then: V
pin
5 = 0.075 I
pk
D SENSEFET
S
K
EA
1.0V
5(9)
+ 0.33 x 10
−3
+1
R
1
R
2
R
1
+ R
2
R
S
1/4 W
Power Ground
To Input Source
Return
R
S
Where: 0
≤
V
Clamp
≤
1.0 V
1−
V
C
3V
Clamp
C
R
1
R
2
R
1
+ R
2
t
Softstart
= − In
Virtually lossless current sensing can be achieved with the implement 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 20 and 22.
Figure 22. Adjustable Buffered Reduction of
Clamp Level with Soft−Start
V
CC
7(12)
+
−
Figure 23. Current Sensing Power MOSFET
V
in
5.0V
ref
+
−
+
−
7(11)
Q1
+
−
T
−
+
Q
R
Comp/Latch
S
6(10)
5(8)
R
3(5)
C
R
S
The addition of the RC filter will eliminate
instability caused by the leading edge spike on
the current waveform.
Figure 24. Current Waveform Spike Suppression
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