UC3842B, UC3843B, UC2842B, UC2843B, NCV3843BV
V
CC
V
in
7(12)
5.0V Ref
8(14)
R
+
−
Bias
5.0V Ref
R
7(11)
6(10)
+
−
8(14)
R
R
Q1
Bias
Osc
4(7)
V
+
Clamp
+
−
S
R
1.0 mA
2R
Q
Osc
5(8)
3(5)
4(7)
2(3)
EA
2(3)
1(1)
R
+
Comp/Latch
1.0V
5(9)
R
2
S
R
1.0mA
Q
R
S
2R
R
1
C
MPSA63
R
1.0V
EA
1.0M
1.67
Where: 0 ≤ V
≤ 1.0 V
V
ꢀ[ꢀ
Clamp
Clamp
R
R
2
ǒ
) 1Ǔ
1(1)
+ * Inƪ11 *ꢀ
ƫꢀC
C
V
V
R ꢀR
1 2
Clamp
R
S
C
5(9)
t
I
ꢀ [ꢀ
Soft-Start
pk(max)
t
≈ 3600C in mF
Soft−Start
R
ꢀ)ꢀ R
3ꢀV
Clamp
1
2
Figure 24. Soft−Start Circuit
Figure 25. Adjustable Buffered Reduction of
Clamp Level with Soft−Start
V
CC
V
in
R
I
r
S pk DS(on)
) R
V
CC
V
in
V
[
(12)
Pin 5
r
DM(on)
S
If: SENSEFET = MTP10N10M
= 200
7(12)
R
S
5.0V Ref
Then :ꢀ V
ꢀ [ꢀ 0.075ꢀI
pk
Pinꢀ5
+
5.0V Ref
−
+
−
D
SENSEFET
(11)
(10)
+
−
7(11)
S
K
+
−
G
Q1
M
6(10)
5(8)
S
R
Q
S
R
(8)
(5)
Q
Comp/Latch
Power Ground:
To Input Source
Return
3(5)
R
Comp/Latch
R
1/4 W
S
C
R
S
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,
reduction of the I clamp level must be implemented. Refer to Figures 23 and 25.
a
The addition of the RC filter will eliminate instability caused by the leading
edge spike on the current waveform.
pk(max)
Figure 26. Current Sensing Power MOSFET
Figure 27. Current Waveform Spike Suppression
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