UC3844B, UC3845B, UC2844B, UC2845B
V
CC
V
in
7(12)
5.0V Ref
8(14)
R
R
5.0V Ref
Bias
8(14)
R
R
+
-
Bias
+
-
7(11)
6(10)
+
-
Osc
Q1
4(7)
T
Osc
+
4(7)
T
S
R
1.0mA
V
+
Clamp
2(3)
1.0M
1(1)
Q
R
R
2
S
R
1.0 mA
2R
2R
R
Q
EA
5(8)
3(5)
EA
2(3)
1(1)
R
1.0V
Comp/Latch
1.0V
R
S
C
5(9)
1
5(9)
t
≈ 3600C in mF
Soft-Start
1.67
2
1
R R
1 2
R ) R
1 2
Where: 0 ≤ V
≤ 1.0 V
-3
ǒ
Ǔ
Clamp
V
Clamp
≈
+ 0.33x10
R
R
ǒ
) 1Ǔ
V
Clamp
R
I ꢁ [ꢁ
pk(max)
S
Figure 20. Adjustable Reduction of Clamp Level
Figure 21. Soft−Start Circuit
V
CC
V
in
V
CC
V
in
R
ꢁI ꢁr
S pk DS(on)
V ꢁ [ꢁ
Pinꢁ5
(12)
7(12)
r
ꢁ )ꢁ R
S
DM(on)
If: SENSEFET
R
=
=
MTP10N10M
200
S
5.0V Ref
5.0V Ref
Then :ꢁ V
ꢁ [ꢁ 0.075ꢁI
pk
Pinꢁ5
8(14)
+
-
R
R
+
-
Bias
D
SENSEFET
(11)
(10)
7(11)
6(10)
+
-
+
-
S
K
G
Q1
Osc
T
4(7)
T
M
+
V
Clamp
S
R
S
R
1.0 mA
2R
Q
Q
(8)
(5)
5(8)
3(5)
EA
2(3)
1(1)
R
Comp/Latch
Comp/Latch
1.0V
5(9)
Power Ground:
To Input Source
Return
R
2
R
1/4 W
S
R
S
R
1
Control Circuitry Ground:
To Pin (9)
MPSA63
1.67
2
1
Where: 0 ≤ V
≤ 1.0 V
V
≈
Clamp
Clamp
Virtually lossless current sensing can be achieved with the implementation
of a SENSEFETt power switch. For proper operation during over-current
R
R
ǒ
) 1Ǔ
conditions, a reduction of the I
Refer to Figures 20 and 22.
clamp level must be implemented.
pk(max)
R ꢁR
1
V
2
V
Clamp
R
S
C
Clamp
+ * Inƪ1 *ꢁ
ƫꢁC
t
I
ꢁ [ꢁ
SoftStart
pk(max)
3V
R ) R
2
1
Figure 22. Adjustable Buffered Reduction of
Figure 23. Current Sensing Power MOSFET
Clamp Level with Soft−Start
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