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PRODUCT SPECIFICATION
RC5031
9
P
Applications Information
The following discussion is intended to be an abbreviated list
of design considerations regarding the RC5031 as used in a
typical voltage processor motherboard application.
For a more thorough discussion of applicable specifications
relating to the Intel Pentium P55C processor, please refer to
Application Note 48.
Output Voltage Selection
Feedback Voltage Divider
The RC5031 precision reference is trimmed to be 1.5V nom-
inally. When using the RC5031, the system designer has
complete flexibility in choosing the output voltage for each
regulator from 1.5V to 3.6V. This is done by appropriately
selecting the feedback resistors. These should be 0.1% resis-
tors to realize optimum output accuracy. The following
equations determine the output voltages of the two regula-
tors:
Switching Regulator
where: R2 > 1.5k
W
and (R2 + R3)
£
25k
W
Example:
For 2.8V,
Short Circuit Considerations
The RC5031 uses a current sensing scheme to limit the load
current if an output fault condition occurs. The current sense
resistor carries the peak current of the inductor, which is
greater than the maximum load current due to ripple currents
flowing in the inductor. The RC5031 will begin to limit the
output current to the load by turning off the top-side FET
driver when the voltage across the current-sense resistor
exceeds the short circuit comparator threshold voltage (V
th
).
When this happens the output voltage will temporarily go
out of regulation. As the voltage across the sense resistor
becomes larger, the top-side MOSFET will continue to turn
off until the current limit value is reached. At this point, the
RC5031 will continuously deliver the limit current at a
reduced output voltage level. The short circuit comparator
threshold voltage is typically 90mV, with a variability of
±
10mV. The ripple current flowing through the inductor is
typically 0.5A. There needs to be a 29% margin for the
sense resistor when using a motherboard PC trace resistor.
Refer to Application Note 48 for detailed discussions. The
sense resistor value can be approximated as follows:
Where TF = Tolerance Factor for the sense resistor and 1.5A
accounts for the inductor ripple current.
There are several different types of sense resistors. Table 3
describes the tolerance, size, power capability, temperature
coefficient and cost of various types of sense resistors.
Based on the Tolerance in Table 3:
For an embedded PC trace resistor:
For a discrete resistor:
V
OUT
1.5
R2
R3
+
R3
è
′
=
V
OUT
1.5
R2
R3
+
R3
è
′
1.5
1.6k
1.85k
+
1.85k
è
′
2.8V
=
=
=
R
SENSE
V
PK
----I
1
TF
–
(
)
′
V
LOAD,MAX
1.5A
1
TF
–
(
)
′
=
=
R
SENSE
V
LOAD,MAX
1.5
1
TF
–
(
)
′
=
1
TF
–
(
)
′
10A
+
1.5A
1
29%
–
(
)
′
4.9m
W
=
=
R
SENSE
V
LOAD,MAX
1.5
1
TF
–
(
)
′
=
1
TF
–
(
)
′
10A
+
1.5A
1
5%
–
(
)
′
6.6m
W
=
=
Table 3. Comparison of Sense Resistors
Motherboard
Trace Resistor
±
29%
Discrete
Iron Alloy
resistor
(IRC)
±
5%
(
±
1% available)
0.45" x 0.065" x
0.2"
1 watt (3 and 5
watts available)
+30 ppm
Discrete
Metal Strip
surface mount
resistor
(Dale)
±
1%
Discrete
MnCu Alloy
wire
resistor
±
10%
Discrete
CuNi Alloy
wire resistor
(Copel)
±
10%
Tolerance Factor
(TF)
Size
(L x W x H)
Power capability
2" x 0.2" x 0.001”
(1 oz Cu trace)
>50A/in
0.25" x 0.125" x
0.025"
1 watt (3 and 5
watts available)
±
75 ppm
0.2" x 0.04" x
0.16"
1 watt
0.2" x 0.04" x
0.1"
1 watt
Temperature
Coefficient
Cost@10,000
piece quantity
+4,000 ppm
±
30 ppm
±
20ppm
Low; included in
motherboard
$0.31
$0.47
$0.09
$0.09