Fairchild SEMICONDUCTOR RC5051 User Manual
Page 11
APPLICATION NOTE
AN50
11
Therefore, for load current of 14.5A, the peak current
through the inductor, I
pk
, is found to be approximately
15.5A:
Therefore, the short circuit detection threshold must be at
least 16.5A.
The next step is to determine the value of the sense resistor.
Including sense resistor tolerance, the sense resistor value
can be approximated as follows
Where TF = Tolerance Factor for the sense resistor.
Table 7 describes tolerance, size, power capability, tempera-
ture coefficient and cost of various type of sense resistors.
I
SC
I
inductor
≥
I
Load, max
I
PK
I
min
–
(
)
2
-----------------------------
+
14.5
2
+
16.5A
=
=
=
R
SENSE
V
th,min
I
SC
----------------
1
TF
–
(
)
×
V
th,min
1.0
I
Load,max
+
-----------------------------------
1
TF
–
(
)
×
=
=
Table 7. Comparison of Sense Resistors
Refer to Appendix A for Directory of component suppliers
Description
Motherboard
Trace Resistor
Discrete Iron
Alloy
Resistor (IRC)
Discrete Metal
Strip Surface
Mount Resistor
(Dale)
Discrete MnCu
Alloy Wire
Resistor
Discrete
CuNi Alloy
Wire Resistor
(Copel)
Tolerance
Factor (TF)
±
29%
±
5%
(
±
1% available)
±
1%
±
10%
±
10%
Size
(L x W x H)
2" x 0.2" x 0.001"
(1 oz Cu trace)
0.45" x 0.065" x
0.200"
0.25" x 0.125" x
0.025"
0.200" x 0.04" x
0.160"
0.200" x 0.04" x
0.100"
Power capability
>50A/in
1 watt
(3W and 5W
available)
1 watt
1 watt
1 watt
Temperature
Coefficient
+4,000 ppm
+30 ppm
±
75 ppm
±
30 ppm
±
20 ppm
Cost
@10,000 piece
Low included in
motherboard
$0.31
$0.47
$0.09
$0.09
Based on the Tolerance Factor in the above table, for an
embedded PC trace resistor and for I
load,max
= 14.5A:
For a discrete resistor and for I
load, max
= 14.5A:
For user convenience, Table 8 lists the recommended values
for sense resistors for various load currents using embedded
PC trace resistors and discrete resistors.
Table 8. R
sense
for Various Load Currents
Discrete Sense Resistor
Discrete Iron Alloy resistors come in variety of tolerances
and power ratings, and are most ideal for precision imple-
mentation. MnCu Alloy wire resistors or CuNi Alloy wire
resistors are ideal for low cost implementations.
R
SENSE
V
th,min
2.0A
I
Load, max
+
----------------------------------------
1
TF
–
(
)
×
=
=
100mV
2.0A
14.5A
+
---------------------------------
1
29%
–
(
)
×
4.3m
Ω
=
R
SENSE
V
th,min
2.0A
I
Load, max
+
----------------------------------------
1
TF
–
(
)
×
=
=
100mV
2.0A
14.5A
+
---------------------------------
1
5%
–
(
)
×
5.8m
Ω
=
I
Load,max
(A)
R
SENSE
PC Trace
Resistor (m
Ω
)
R
SENSE
Discrete
Resistor (m
Ω
)
10.0
5.9
7.9
11.2
5.4
7.2
12.4
4.9
6.6
13.9
4.5
6.0
14.0
4.4
5.9
14.5
4.3
5.8