UC3842B, 43B UC2842B, 43B
Figure 24. Adjustable Buffered Reduction of
Figure 25. Current Sensing Power MOSFET
Clamp Level with Soft–Start
V
V
in
CC
V
V
in
CC
R
I
r
pk DS(on)
S
V
(12)
Pin 5
7(12)
r
R
S
DM(on)
If: SENSEFET = MTP10N10M
= 200
R
S
5.0V Ref
5.0V Ref
Then :
V
0.075 I
pk
Pin
5
8(14)
4(7)
+
–
+
–
R
Bias
D
SENSEFET
R
7(11)
6(10)
(11)
(10)
+
–
+
–
Q1
S
K
G
Osc
V
+
Clamp
M
S
R
1.0 mA
2R
S
R
Q
Q
5(8)
3(5)
EA
(8)
(5)
2(3)
1(1)
R
Comp/Latch
1.0V
5(9)
Comp/Latch
R
2
Power Ground:
To Input Source
Return
R
R
S
S
R
1/4 W
C
1
MPSA63
Control Circuitry Ground:
To Pin (9)
1.67
Where: 0
≤
V
R
≤ 1.0 V
V
Clamp
Clamp
R
R
2
1
1
Virtually lossless current sensing can be achieved with the implementation of
SENSEFET power switch. For proper operation during over–current conditions,
a
a
reduction of the I clamp level must be implemented. Refer to Figures 22 and 24.
V
V
V
R
C
Clamp
R
S
pk(max)
C
1
2
R
2
t
In
1
I
pk(max)
Soft-Start
R
3
1
Clamp
Figure 26. Current Waveform Spike Suppression
Figure 27. MOSFET Parasitic Oscillations
V
V
V
V
in
CC
in
CC
7(12)
7(12)
5.0V Ref
5.0V Ref
+
–
+
–
7(11)
7(11)
+
–
+
–
R
Q1
Q1
g
6(10)
5(8)
6(10)
S
R
S
R
Q
Q
5(8)
3(5)
3(5)
R
Comp/Latch
Comp/Latch
C
R
S
R
S
Series gate resistor R will damp any high frequency parasitic oscillations
g
The addition of the RC filter will eliminate instability caused by the leading
edge spike on the current waveform.
caused by the MOSFET input capacitance and any series wiring inductance in
the gate–source circuit.
12
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
