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| 型号： | AN-53 |
| 厂商： | Fairchild Semiconductor Corporation |
| 元件分类： | DC/DC变换器 |
| 英文描述： | Implementing an RC5051 DC-DC Converter on Pentium II Motherboards |
| 中文描述： | 推行RC5051的DC - DC转换器的奔腾II主板 |
| 文件页数： | 12/20页 |
| 文件大小： | 143K |
| 代理商： | AN-53 |
AN53
APPLICATION NOTE
12
Thermal Consideration.
Due to I
2
R power losses the temperature of the
resistor will
increase, leading to a higher resistance value. In addition,
ambient temperature variation will add to the change in
resistor value:
1.00393
T
′
=
where: R
20C
is the resistance at 20
°
C, T is the trace’s
temperature, and R is the actual resistance. For example,
for temperature T = 50
°
C, the %R change = 12%. Table
10 is the summary of the tolerance for the Embedded PC
Trace Resistor.
Table 10. Summary PC Trace Resistor
Tolerance
Design Rules for Using an Embedded Resistor
The basic equation for the resistance of an embedded
resistor is:
where:
r
= Resistivity(
mW
-mil),
L = Length(mils),
W = Width(mils), and
t = Thickness(mils).
For 1oz copper, t = 1.3 mils,
r
= 677
mW
-mil,
1 L/1 W = 1 Square (
I
I
).
For example, you can layout a 5.20m
W
embedded sense
resistor using the example above
L/W = 300mils
L/W = 10
I
I
Refer to Figure 8.
Figure 8. 5.20m
W
Sense Resistor (10
I
I
)
You can also implement the sense resistor in the following
manner. Each corner square is counted as 0.6 square since
current flowing through the corner square does not flow
uniformly and it is concentrated towards the inside edge,
as shown in Figure 9.
Figure 9. 5.30m
W
Sense Resistor (10
I
I
)
A Design Example Combining an Embedded Resistor
and a Discrete Resistor
For low cost implementation, the embedded PC trace resistor
is most desirable. However, its wide tolerance (20%) can
present a problem. In addition, requirements for the CPU
change frequently, and, thus, the maximum load current may
be subject to change. Combining embedded resistors with
discrete resistors may be a desirable option. Figure 10 shows
a design that provides flexibility with a solution to address
wide tolerances. In this design, you have the option to
choose an embedded or a discrete sense resistor. To use the
discrete sense resistor, populate R21 with a shorting bar
(zero Ohm resistor) for proper Kelvin connection and add
the MnCu sense resistor. To use the embedded sense resistor,
on the other hand, populate R22 with a shorting bar for
Kelvin connection. The embedded sense resistor allows the
user to choose a plus or a minus delta resistance tap to offset
any large sheet resistivity change. In this design, the center
tap yields 6m
W
, the left tap yields 6.8m
W
, and the right tap
yields 5.3m
W
.
Tolerance due to Sheet Resistivity variation
8%
Tolerance due to temperature variation
12%
Total Tolerance for PC Trace Resistor
20%
R
R
20C
20
–
R
r
t
′
W
′
=
L
r
RWt
0.0052
300
677
′
1.3
′
3000mils
=
=
=
1 Each
W = 300 mils
L = 3000mils
65-AP53-05
1 Each
.6
.8
.6
65-AP53-06
Figure 10. Short Circuit Sense Resistor Design Using a PC Trace Resistor and an Optional Discrete Sense Resistor
IFB
VFB
MnCu Discrete
Resistor
R21
R22
Output Power
Plane (Vout)
R-
D
r
R
R+
D
r
65-AP53-07
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