UC3844, UC3845, UC2844, UC2845
VCC
7(12)
Vin
8(14)
4(7)
R2
2(3)
1(1)
R1
R
Bias
R
OSC
+
+−
EA
1.0mA
2R
R
5.0Vref
+− +
+
−
+−
−
VClamp
−
+
1.0V
T
S
Q
R
Comp/Latch
5(9)
7(11)
Q1
6(10)
5(8)
3(5)
RS
VClamp
1.67
R2 + 1
R1
+ 0.33 x 10−3
R1 R2
R1 + R2
Ipk(max) ≈
VClamp
RS
Where: 0 ≤ VClamp ≤ 1.0 V
Figure 20. Adjustable Reduction of Clamp Level
8(14)
R
Bias
R
5.0Vref
+ +−
−
OSC
4(7)
+
T
+
1.0mA
−
2(3)
EA
2R
R
S
−+
Q
R
1.0V
C
1(1)
5(9)
tSoft−Start 3600C in mF
Figure 21. Soft−Start Circuit
VCC
7(12)
Vin
8(14)
R
Bias
R
5.0Vref
+ +−
+− +
−
4(7)
2(3)
R2
1(1)
OSC
+
+−
EA
1.0mA
2R
R
−
VClamp
−+
1.0V
T
S
RQ
Comp/Latch
C
R1
MPSA63
VClamp
Ipk(max)≈
VClamp
RS
tSoftstart = − In
1.67
R2 + 1
R1
5(9)
+ 0.33 x 10−3
R1 R2
R1 + R2
Where: 0 ≤ VClamp ≤ 1.0 V
1−
VC
3VClamp
C
R1 R2
R1 + R2
7(11)
Q1
6(10)
5(8)
3(5)
RS
5.0Vref
+ +−
−
VCC
(12)
+− +
−
(11)
Vin
VPin 5 ≈
RS Ipk rDS(on)
rDM(on) + RS
If: SENSEFET = MTP10N10M
RS = 200
Then: Vpin 5 = 0.075 Ipk
D SENSEFET
S
T
S
−+ R Q
Comp/Latch
Control CIrcuitry
Ground:
To Pin (9)
(10) G M
(8)
(5) RS
1/4 W
K
Power Ground
To Input Source
Return
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
Ipk(max) clamp level must be implemented. Refer to Figures 20 and 22.
Figure 22. Adjustable Buffered Reduction of
Clamp Level with Soft−Start
VCC
7(12)
Figure 23. Current Sensing Power MOSFET
Vin
5.0Vref
+
−+
+ +−
−
−
T
S
−
+
RQ
Comp/Latch
7(11)
Q1
6(10)
5(8)
R
3(5)
C
RS
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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