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RC5042
PRODUCT SPECIFICATION
14
where: V
in
= input voltage to Converter
V
SW
= voltage across Switcher (MOSFET) =
I
LOAD
x R
DS(ON)
V
D
= Forward Voltage of the Schottky diode
T = the switching period of the converter = 1/f
S
,
where f
S
= switching frequency.
For an input voltage of 5V, an output voltage of 3.3V, an
inductor value of 1.3
m
H and a switching frequency of
650KHz (using C
EXT
=39pF), the inductor current can be
calculated as follows:
I
---------------------------
I
–
2
(
)
-5.0
14.5
1.3
0.037
10
′
–
3.3
–
(
)
6
–
′
′
=
0.5
0.037
+
′
)
14.5
–
0.5
+
(
)
-5.0
650
10
3
′
---------1
′
1.048A
=
Therefore, the peak current, I
PK
, through the inductor for a
14.5A load is found to be:
As a result, the short circuit detection threshold must be at
least 15.5A.
The next step is to determine the value of the sense resistor.
Including sense resistor tolerance, the sense resistor value
can be approximated as follows:
Where TF = Tolerance Factor for the sense resistor.
There are several different type of sense resistors. Table 7
describes tolerance, size, power capability, temperature
coefficient and cost of various type of sense resistors:
I
SC
I
inductor
3
I
Load, max
I
------------2
I
–
(
)
+
14.5
1
+
15.5A
=
=
=
R
SENSE
V
I
SC
----------------
1
TF
–
(
)
′
V
I
Load,max
+
1.0
----------------------------------
1
TF
–
(
)
′
=
=
Table 7. Comparison of Sense Resistors
1
Description
Tolerance
Factor (TF)
Motherboard
Trace Resistor
±
29%
Discrete Iron
Alloy
resistor (IRC)
±
5%
(
±
1%
available)
0.45" x 0.065" x
0.200"
1 watt
(3W and 5W
available)
+30 ppm
Discrete Metal
Strip surface
mount resistor
(Dale)
±
1%
Discrete MnCu
Alloy wire
resistor
±
10%
Discrete
CuNi Alloy
wire resistor
(Copel)
±
10%
Size
(L x W x H)
Power capability >50A/in
2" x 0.2" x 0.001"
(1 oz Cu trace)
0.25" x 0.125" x
0.025"
1 watt
0.200" x 0.04" x
0.160"
1 watt
0.200" x 0.04" x
0.100"
1 watt
Temperature
Coefficient
Cost
@10,000 piece
+4,000 ppm
±
75 ppm
±
30 ppm
±
20 ppm
Low
included in
motherboard
$0.31
$0.47
$0.09
$0.09
Notes:
1. Refer to Appendix A for Directory of component suppliers
Based on the Tolerance in the above table,
For Embedded PC Trace Resistor
and for
I
l
oad,max
= 14.5A
:
V
I
Load, max
+
For discrete resistor and I
load, max
= 14.5A
:
V
I
Load, max
+
R
SENSE
1.0A
1
TF
–
(
)
′
=
=
14.5A
+
1.0A
1
29%
–
(
)
′
4.6m
W
=
R
SENSE
1.0A
1
TF
–
(
)
′
=
=
14.5A
+
1.0A
1
5%
–
(
)
′
6.1m
W
=
For user convenience, Table 8 lists recommended value for
sense resistor for various load current using embedded PC
trace resistor or discrete resistor.
Table 8. R
sense
for Various Load Current
I
Load,max
(A)
10.00
11.20
12.40
13.90
14.00
14.50
R
SENSE
PC Trace
Resistor (m
W
)
6.5
5.8
5.3
4.8
4.7
4.6
R
SENSE
Discrete
Resistor (m
W
)
8.6
7.8
7.1
6.4
6.3
6.1