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RC5041
PRODUCT SPECIFICATION
12
P
Therefore, the peak current, I
PK
, through the inductor for a
14.5A load is found to be:
As a result, the short circuit detection threshold must be at
least 15.5A
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.
I
R
= Ripple Current = 1A
There are several different type of sense resistors. Table 7
describes tolerance, size, power capability, temperature
coefficient and cost of various type of sense resistors:
Based on the Tolerance in Table 5,
For Embedded PC Trace Resistor
and for
I
l
oad,max
= 14.5A
:
V
2.0A
I
Load, max
+
I
-----------2
I
–
(
)
-5.0
14.5
1.3
0.037
10
×
×
–
3.3
–
(
)
6
–
×
=
0.5
0.037
+
)
14.5
×
–
0.5
+
)
-5.0
×
650
10
3
----------1
×
1.048A
=
I
SC
I
inductor
≥
I
Load, max
I
-------------2
I
–
(
)
+
14.5
1
+
15.5A
=
=
=
R
SENSE
V
+
I
SC
1
1
TF
–
(
)
×
V
Load,max
I
R
+
+
1.0
1
TF
–
(
)
×
=
=
R
SENSE
----------------------------------------
1
TF
–
(
)
×
=
=
2.0A
14.5A
+
------100mV
1
29%
–
(
)
×
4.3m
=
For discrete resistor and I
load, max
= 14.5A
:
V
I
Load, max
+
For user convenience, Table 6 lists recommended value for
sense resistor for various load current using embedded PC
trace resistor or discrete resistor.
Table 6. R
sense
for Various Load Current
I
Load,max
(A)
10.00
11.20
12.40
13.90
14.00
14.50
R
SENSE
PC Trace
Resistor (m
)
5.9
5.4
4.9
4.5
4.4
4.3
R
SENSE
Discrete
Resistor (m
)
7.9
7.2
6.6
6.0
5.9
5.7
R
SENSE
I
R
+
1.0A
1
TF
–
(
)
×
=
=
14.5A
+
2.0A
1
5%
–
(
)
×
5.75m
=
Table 5. Comparison of Sense Resistors
1
Notes:
1.Refer to Appendix A for Directory of component suppliers.
Description
Tolerance
Factor (TF)
Size
(L x W x H)
Power capability >50A/in
Motherboard
Trace Resistor
±
29%
Discrete Iron
Alloy
resistor (IRC)
±
5%
(
±
1% available)
0.45" x 0.065" x
0.200"
1 watt
(3W and 5W
available)
+30 ppm
Discrete Metal
Strip surface
mount resistor
(Dale)
±
1%
Discrete MnCu
Alloy wire
resistor
±
10%
Discrete
CuNi Alloy
wire resistor
(Copel)
±
10%
2" x 0.2" x 0.001"
(1 oz Cu trace)
0.25" x 0.125" x
0.025"
1 watt
0.200" x 0.04" x
0.160"
1 watt
0.200" x 0.04" x
0.100"
1 watt
Temperature
Coefficient
Cost
@10,000 piece
+4,000 ppm
±
75 ppm
±
30 ppm
±
20 ppm
Low
included in
motherboard
$0.31
$0.47
$0.09
$0.09