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| 型號(hào)： | AN-53 |
| 廠(chǎng)商： | Fairchild Semiconductor Corporation |
| 元件分類(lèi)： | DC/DC變換器 |
| 英文描述： | Implementing an RC5051 DC-DC Converter on Pentium II Motherboards |
| 中文描述： | 推行RC5051的DC - DC轉(zhuǎn)換器的奔騰II主板 |
| 文件頁(yè)數(shù)： | 11/20頁(yè) |
| 文件大?。?/td> | 143K |
| 代理商： | AN-53 |
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AN53
APPLICATION NOTE
11
Table 8. Comparison of Sense Resistors
Continuing with the example, based on the Tolerance Factor
in the above table, for an embedded PC trace resistor and for
I
SC
set 1A greater than Ip
k
:
For a discrete Cuni resistor
:
For user convenience, Table 9 lists the recommended values
for sense resistors for various load currents using embedded
trace resistors and discrete resistors and assuming (I
pk
- I
min
) /
2 = 10% I
OUT
and I
SC
set 1A greater than I
pk
Table 9. R
sense
for Various Load Currents
Comment on Discrete Sense Resistors
Discrete Iron Alloy resistors come in a variety of tolerances
and power ratings, and are ideal for precision implementa-
tion, minimizing stress on the MOSFETs. MnCu Alloy wire
resistors or CuNi Alloy wire resistors are ideal for low cost
implementations.
Designing an Embedded Sense Resistor (PC Trace
Resistor)
Embedded PC trace resistors have the advantage of near zero
cost implementation. However, the value of the PC trace
resistance has large variations. To start with, traces on the
outside layers have far more variation than those on inside
layers; for this reason, embedded resistors should always be
designed on inside layers. Embedded resistors have 3 error
sources: the sheet resistivity of the layer, the tolerance of the
width and length of the trace, and the temperature variation
of the copper. Only two of the error sources must be consid-
ered for laying out embedded sense resistors.
Sheet resistivity.
For 1 ounce copper, the thickness is nominally 1.3mils,
and the toleranace is typically
±
.1 mil. Therefore error due
to sheet resistivity is 0.1 mil / 1.3mil = +/-8%
Tolerance of width and length.
The width and length of traces also have tolerances;
typical numbers might be
±
3/4 mil. A typical width to
obtain a 5.2m
W
resistor might be 300mils, and so the
length would have to be:
w
′
R
′
r
6.77
10
′
or 3 inches. The tolerance is thus negligible.
Description
Tolerance Factor
(TF)
Size (L x W x H)
Motherboard
Trace Resistor
±
20%
Discrete Iron
Alloy
Resistor (IRC)
±
5%
(
±
1% available)
0.45" x 0.065" x
0.200"
1 watt
(3W and 5W
available)
+390 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)
>50A/in
0.25" x 0.125" x
0.025"
1 watt
(2W available)
0.200" x 0.04" x
0.160"
>1 watt
0.200" x 0.04" x
0.100"
>1 watt
Power capability
Temperature
Coefficient
Cost @10,000
piece
+3900 ppm
±
75 ppm
±
50 ppm
±
20 ppm
Low---included in
motherboard
$0.31
$0.47
$0.09
$0.09
I
Load,max
(A)
6.9
7.8
8.5
8.7
9.6
10.6
11.1
12.6
14.2
17.2
18.5
18.9
R
SENSE
PC Trace
Resistor (m
W
)
9.7
8.7
8.1
7.9
7.2
6.6
6.3
5.6
5.0
4.2
3.9
3.8
R
SENSE
Discrete
Resistor (m
W
)
10.6
9.5
8.8
8.6
7.9
7.2
6.9
6.1
5.5
4.6
4.3
4.2
R
SENSE
(
)
1
(
0.20
+
)
-15.7A
5.0m
W
=
=
R
SENSE
1A
+
15.7A
(
)
1
(
0.10
+
)
--------------------100mV
5.7m
W
=
=
L
t
1.3mil
300mil
′
5.2m
W
′
–
4
–
W
mil
2996mils
=
=
=
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