UC3844, 45 UC2844, 45
Figure 19. Adjustable Reduction of Clamp Level
Figure 20. Soft–Start Circuit
V
CC
V
in
7(12)
5.0V
+
–
ref
5.0V
ref
8(14)
R
R
+
8(14)
R
R
Bias
+
–
–
Bias
+
+
–
+
7(11)
6(10)
5(8)
–
–
Q1
Osc
Osc
T
+
4(7)
2(3)
T
+
4(7)
V
Clamp
S
1.0mA
2R
+
–
1.0mA
2R
S
Q
–
+
–
Q
R
–
+
R2
R
EA
+
EA
2(3)
1(1)
R
Comp/Latch
R
1.0V
1.0V
3(5)
1(1)
t
C
R
S
R1
5(9)
5(9)
3600C in µF
Soft–Start
1.67
2
R
R
2
V
1
Clamp
–3
+ 0.33 x 10
V
Clamp
I
pk(max)
≈
R
+ R
R
1
2
S
R
R
+ 1
Where: 0
≤
V
≤
1.0 V
Clamp
1
Figure 21. Adjustable Buffered Reduction of
Figure 22. Current Sensing Power MOSFET
Clamp Level with Soft–Start
V
CC
R
I
r
S
pk DS(on)
V
V
in
CC
V
5 ≈
V
(12)
Pin
in
r
+ R
DM(on)
S
7(12)
If: SENSEFET = MTP10N10M
+
–
R
= 200
5.0V
ref
S
+
–
+
5.0V
ref
Then: V
SENSEFET
5 = 0.075 I
pk
+
pin
8(14)
4(7)
R
R
–
+
D
–
Bias
–
+
+
–
(11)
(10)
(8)
+
S
7(11)
6(10)
5(8)
–
–
Q1
Osc
G
T
K
T
M
+
V
Clamp
S
S
Q
1.0mA
2R
–
+
–
R
Q
+
–
R
Power Ground
To Input Source
Return
+
Comp/Latch
EA
2(3)
1(1)
Comp/Latch
R
R2
(5)
R
1.0V
3(5)
S
1/4 W
R
S
Control CIrcuitry
Ground:
To Pin (9)
5(9)
–3
MPSA63
C
R1
1.67
2
R
R
2
V
1
Clamp
+ 0.33 x 10
R
+ R
R
R
1
2
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
+ 1
1
V
I
clamp level must be implemented. Refer to Figures 19 and 21.
Clamp
pk(max)
I
≈
Where: 0
V
≤
V
≤ 1.0 V
pk(max)
Clamp
C
R
S
R
R
C
1
2
t
= – In
1 –
Softstart
3V
R
+ R
Clamp
1 2
Figure 23. Current Waveform Spike Suppression
V
CC
V
in
7(12)
+
5.0V
ref
–
+
–
+
–
+
7(11)
6(10)
5(8)
–
Q1
T
S
Q
–
R
+
R
Comp/Latch
3(5)
The addition of the RC filter will eliminate
instabilitycaused by the leading edge spike on
the current waveform.
C
R
S
10
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
