UC3844, 45 UC2844, 45
Figure 19. Adjustable Reduction of Clamp Level
VCC
7(12)
+
–
Vin
Figure 20. Soft–Start Circuit
5.0Vref
8(14)
R
Bias
R
Osc
4(7)
R2
2(3)
1(1)
5(9)
1.67
R2
+1
R1
+ 0.33 x 10–3
R1 R2
R1 + R2
+
–
EA
+
1.0mA
2R
R
–
+
1.0V
R1
+
–
VClamp
T
+
–
5.0Vref
+
–
7(11)
Q1
6(10)
1.0M
5(8)
3(5)
RS
4(7)
+
–
2(3)
1(1)
tSoft–Start
EA
+
1.0mA
2R
R
–
+
1.0V
C
Osc
8(14)
R
Bias
R
+
–
T
S
R
Q
+
–
Q
R
Comp/Latch
S
3600C in
µF
5(9)
VClamp
Ipk(max)
≈
VClamp
RS
Where: 0
≤
VClamp
≤
1.0 V
Figure 21. Adjustable Buffered Reduction of
Clamp Level with Soft–Start
VCC
7(12)
+
–
Vin
Figure 22. Current Sensing Power MOSFET
VCC
(12)
+
–
RS Ipk rDS(on)
VPin 5
≈
rDM(on) + RS
If: SENSEFET = MTP10N10M
RS = 200
Then: Vpin 5 = 0.075 Ipk
D SENSEFET
Vin
S
M
K
5.0Vref
8(14)
R
Bias
R
Osc
4(7)
+
–
2(3)
R2
1(1)
C
MPSA63
R1
VClamp
1.67
R2
+1
R1
5(9)
+ 0.33 x 10–3
R1 R2
R1 + R2
+
1.0mA
EA
2R
R
+
–
VClamp
T
+
–
5.0Vref
+
–
7(11)
Q1
6(10)
5(8)
3(5)
RS
–
+
+
–
T
+
–
+
–
(11)
(10) G
(8)
(5)
S
Q
–
R
+
Comp/Latch
1.0V
S
Q
R
Comp/Latch
Control CIrcuitry
Ground:
To Pin (9)
RS
1/4 W
Power Ground
To Input Source
Return
Ipk(max)
≈
VClamp
RS
Where: 0
≤
VClamp
≤
1.0 V
1–
VC
3VClamp
C
R1 R2
R1 + R2
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 19 and 21.
tSoftstart = – In
Figure 23. Current Waveform Spike Suppression
VCC
7(12)
+
–
Vin
5.0Vref
+
–
+
–
7(11)
Q1
+
–
T
–
+
Q
R
Comp/Latch
S
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.
10
MOTOROLA ANALOG IC DEVICE DATA
ONSEMI [ ON SEMICONDUCTOR ]
