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| 型號： | AN606 |
| 廠商： | Vishay Intertechnology,Inc. |
| 英文描述： | Current-Sensing Power MOSFETs |
| 中文描述： | 電流感應(yīng)功率MOSFET |
| 文件頁數(shù)： | 3/4頁 |
| 文件大?。?/td> | 68K |
| 代理商： | AN606 |
AN606
Vishay Siliconix
Document Number: 71991
17-Dec-03
www.vishay.com
3
FIGURE 3.
Normalized r
m(on)
for the Sense Die
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
50
25
0
T
J
Junction Temperature ( C)
25
50
75
100
125
150
175
On-Resistance vs. Junction Temperature
V
GS
= 10 V
I
D
r
m
N
DESIGN EQUATIONS
The following three equations enable circuit design and
analysis.
I
SENSE
= x I
D
/r
V
DS
= I
SENSE
x [r
m(on)
+ R
SENSE
] or
V
DS
= I
D
x r
DS(on)
/(r
m(on)
+ R
SENSE
)
V
SENSE
= I
SENSE
x R
SENSE
or
V
SENSE
= V
DS
x R
SENSE
/(r
m(on)
+ R
SENSE
)
Where I
SENSE
Current flowing out of sense terminal
Current-sensing ratio
Drain-source current
Drain-source voltage
Mirror active resistance
External current-sense resistor
r
I
D
V
DS
r
m(on)
R
SENSE
Application Aspects and Design Examples
The current-sense ratio r, even though fixed by design, is
dependent
on
manufacturing
Furthermore, mirror active resistance r
m(on)
depends on circuit
parameters V
GS
and I
D
and junction temperature T
J
. As a
result, a practical design can realize an accuracy of 15% —
20% for current sensing. Accordingly, the current-sensing
MOSFET is most suitable for supervisory functions such as
overcurrent and/or short-circuit protection.
process
variations.
Three keys to a successful design are to:
1. have an adequate margin between the normal
operating-current value and the trip-current value;
2. use a minimum value of the I
SENSE
signal at the maximum
value of I
D
; and
3. use a fast comparator with hysterisis to control and protect
the MOSFET.
Typical schematic configurations for implementing the current
sense are shown in Figure 4 and Figure 5.
The Virtual Earth Sensing Scheme, Figure 4, is suitable for
applications aiming at higher noise immunity and speed. This
approach also improves measurement accuracy by
eliminating the sense resistor. However, a dual power supply
and inverted (negative) output signal are the price designers
pay for deriving these benefits.
The Resistor Sensing Scheme shown in Figure 5 is a quite
simple and economical approach. The accuracy of current
measurement is affected by the introduction of an external
sense resistor R
S
. However, the latter aids in lowering the
temperature sensitivity of the current-sense signal.
V
1
G
V
DD
S
Sense
Kelvin
Load
R1
Stray
Resistance
R2
R3
+
D
I
L
G
V
DD
S
Sense
Kelvin
Load
+
V
1
D
I
L
V
+V
FIGURE 4.
Virtual Earth Sensing Scheme
FIGURE 5.
Resistor Sensing Scheme
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