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