
RC10-XX
PRODUCT SPECIFICATION
2
P
System Requirements
The design of the sense resistor is driven by the following
system requirements:
Load current, (I
LOAD
). This is the full load DC current
the converter is designed to support.
The controller short circuit current detect threshold
voltage (V
TH
), which for Fairchild Semiconductor family
of Controllers is specified at 120+/-20mV for the RC5040,
RC5041, RC5042, RC5050, and RC5051, or 90
±
10mV
for the RC5036.
The inductor current ripple (I
R
). A reasonable design
guideline is to assume the current ripple is limited to
1.5A.
Design Equations
The design of the sense resistor must consider carefully the
output requirements during normal operation and during a
fault condition. If the sense resistor is too high, it may
develop enough voltage drop accross it to trip the short cir-
cuit detect circuitry so that the DC-DC converter may not be
able to deliver the maximum required load current. If the
sense resistor is too low, the controller may not be disabled
when a certain safe amount of load current is exceeded, thus
the power dissipation within the MOSFET(s) may rise to
destructive levels.
The design equations used to calculate the sense resistor are
as follows:
and, assuming a 10% tolerance, the nominal design value of
the sense resistor is given by:
Wire Sense Resistors
There are several types of sense resitors available to the
system designer in a wide range of cost and specifications.
The resistors with higher precision (i.e. 1% SMT) demand
the highest cost (i.e. $0.47); however a 10% to 15%
tolerance is adequate for most DC-DC converters designs
and wire resistors offer a very cost effective alternative.
MnCu or CuNi alloy wire resistors have been used
Fairchild Semiconductor’s family of DC-DC converters.
These resistive wires are available in all the most common
gages and Table 1 and Table 2 describe the specifications of
MnCu and CuNi alloys for various wire diameters.
Wire with diameter of ~1 mm is best suited to make sense
resistors for DC-DC converters used in motherboard applica-
tions. Refer to Figure 1 for the typical shape of a wire sense
resistor and Figure 2 through Figure 5 for the dimensions of
the resistor as function of the load current requirements of
the converter.
Table 1. MnCu Wire Resistor Specifications
Note:
1. J = 10
4
A/cm
2
Table 2. CiNi Wire Resistor Specifications
Note:
1. J = 10
4
A/cm
2
I
SC MIN
)
I
LOAD
I
R
1
+
+
(
)
=
R
SENSE MAX
)
V
I
SC MIN
)
)
---------------------------
=
R
SENSE
R
1
+
(
)
0.10
)
---------------------------------------
=
Diameter
(mm)
1.40
1.30
1.10
1.00
0.90
0.80
W
/m
0.31831
0.36916
0.51561
0.62389
0.77023
0.97482
m
W
/mm
0.318
0.369
0.516
0.624
0.770
0.975
Imax
(Amp)
154
133
95
79
64
50
AWG
15
16
17
18
19
20
Diameter
(mm)
1.45
1.29
1.15
1.02
0.912
0.812
W
/ft
m
W
/m
0.30
0.37
0.48
0.60
0.74
0.94
Imax
(Amp)
165
131
104
82
65
52
0.09049
0.11300
0.14520
0.18370
0.22690
0.28710