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ADVANCED INFORMATION
describes products that are in the planning or early design stage. Specifications may change in any manner without
notice. Contact Fairchild Semiconductor for current information.
A
www.fairchildsemi.com
Features
Resistor typical tolerance
±
5%
Resistance wire TCR +20ppm/C
Wire alloy MnCu (mangnin)
Resistor Dimensions
Applications
System Requirements
The design of the sense resistor is driven by the following
system requirements:
1.
Load current, (I
LOAD
). This is the full load DC current
the converter is designed to support.
2.
The controller short circuit current detect threshold volt-
age (V
TH
), which for Fairchild Semiconductor family of
Controllers is specified at 120+/-20mV for RC5040/41/
42/50/51 or 90
±
10mV for RC5036.
3.
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:
R
)
1
0.10
+
(
)
Wire Sense Resistors
There are several types of sense resitors available to the sys-
tem 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% toler-
ance is adequate for most DC-DC converters designs and
wire resistors offer a very cost effective alternative.
Mangnin or Copel wire resistors, made respectively of
MnCu and CuNi alloys, have been used extensively in the
manufacture of sense resistors used in the Fairchild Semi-
conductor’s family of DC-DC converters.
These resistive wires are available in all the most common
gages and Table 1 and 2 describe the specifications of Mang-
nin and Copel for various diameter wire size.
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 Graph 1 through 4 for the dimensions of the
resistor as function of the load current requirements of the
converter.
Type
RC40-58
RC40-44
R (m
W
)
5.8
4.4
D (mm)
1.00
1.00
L (mm)
9.3
7.1
H (mm)
5.0
5.0
Controller Type
RC5036
RC5036
RC5041/RC5042
RC5041/RC5042
RC5050/RC5051
RC5050/RC5051
I
Lmax
(A)
10
14
13
18
13
18
R
sense
Type
RC40-58
RC40-36
RC40-58
RC40-36
RC40-58
RC40-36
D
H
L
I
SC MIN
)
I
LOAD
I
R
1
+
+
(
)
=
R
SENSE MAX
)
V
I
SC MIN
)
)
---------------------------
=
R
SENSE
---------------------------------------
=
RC40-X X
S ense Resistors for Fairc hild S emic onduc tor
DC-DC Controllers
Rev. 0.5.1