AN606 Ver la hoja de datos (PDF) - Vishay Semiconductors
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AN606 Datasheet PDF : 4 Pages
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AN606
Vishay Siliconix
On-Resistance vs. Junction Temperature
2.0
VGS = 10 V
1.8
ID = 25 A
1.6
1.4
1.2
1.0
0.8
0.6
−50 −25
0 25 50 75 100 125 150 175
TJ − Junction Temperature (_C)
FIGURE 3. Normalized rm(on) for the Sense Die
DESIGN EQUATIONS
The following three equations enable circuit design and
analysis.
ISENSE = x ID/r
VDS = ISENSE x [rm(on) + RSENSE] or
VDS = ID x rDS(on)/(rm(on) + RSENSE)
VSENSE = ISENSE x RSENSE or
VSENSE = VDS x RSENSE/(rm(on) + RSENSE)
Where
ISENSE
r
ID
VDS
rm(on)
RSENSE
Current flowing out of sense terminal
Current-sensing ratio
Drain-source current
Drain-source voltage
Mirror active resistance
External current-sense resistor
Application Aspects and Design Examples
The current-sense ratio r, even though fixed by design, is
dependent on manufacturing process variations.
Furthermore, mirror active resistance rm(on) depends on circuit
parameters VGS and ID and junction temperature TJ. 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.
Three keys to a successful design are to:
1. have an adequate margin between the normal
operating-current value and the trip-current value;
Document Number: 71991
17-Dec-03
2. use a minimum value of the ISENSE signal at the maximum
value of ID; 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 RS. However, the latter aids in lowering the
temperature sensitivity of the current-sense signal.
VDD
Load
IL
D
Kelvin
G
Sense
+V
−
+
V1
−V
S
FIGURE 4. Virtual Earth Sensing Scheme
VDD
G
Sense
Load
IL
D
Kelvin
R1
R3
R2
−
+
V1
S
Stray
Resistance
FIGURE 5. Resistor Sensing Scheme
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