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UC2844 View Datasheet(PDF) - ON Semiconductor

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UC2844 Datasheet PDF : 16 Pages
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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. SoftStart 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 SoftStart
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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