RC5042 View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
Manufacturer
RC5042 Datasheet PDF : 20 Pages
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RC5042
PRODUCT SPECIFICATION
where: • Vin = input voltage to Converter
• VSW = voltage across Switcher (MOSFET) =
ILOAD x RDS(ON)
• VD = Forward Voltage of the Schottky diode
• T = the switching period of the converter = 1/fS,
where fS = switching frequency.
For an input voltage of 5V, an output voltage of 3.3V, an
inductor value of 1.3mH and a switching frequency of
650KHz (using CEXT=39pF), the inductor current can be
calculated as follows:
(---I--p---k----–----I--m----i--n---)
2
=
-(--5---.-0-----–----1---4---.--5----´-----0---.--0---3---7----–-----3---.-3----)
1.3 ´ 10–6
´
Therefore, the peak current, IPK, through the inductor for a
14.5A load is found to be:
ISC ³ Iinductor
=
ILoad,
max
+
(---I--P---K-----–----I---m----i-n---)-
2
=
14.5 + 1
=
15.5A
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:
RSENSE
=
V-----t-h---,-m---i--n- ´ (1 – TF)
ISC
=
----------V----t-h---,-m----i-n---------- ´ (1 – TF)
1.0 + ILoad,max
(---5---.-0-----–----1---(4--3-.--5.--3--´--+---0--0-.--0.-5--3--)-7----+-----0---.-5----)-
´
-----------1------------
650 ´ 103
=
1.048A
Table 7. Comparison of Sense Resistors1
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:
Description
Motherboard
Trace Resistor
Discrete Iron
Alloy
resistor (IRC)
Discrete Metal
Strip surface
mount resistor
(Dale)
Discrete MnCu
Alloy wire
resistor
Discrete
CuNi Alloy
wire resistor
(Copel)
Tolerance
Factor (TF)
±29%
±5%
(±1%
available)
±1%
±10%
±10%
Size
(L x W x H)
2" x 0.2" x 0.001" 0.45" x 0.065" x 0.25" x 0.125" x 0.200" x 0.04" x 0.200" x 0.04" x
(1 oz Cu trace) 0.200"
0.025"
0.160"
0.100"
Power capability >50A/in
1 watt
(3W and 5W
available)
1 watt
1 watt
1 watt
Temperature
Coefficient
+4,000 ppm
+30 ppm
±75 ppm
±30 ppm
±20 ppm
Cost
Low
@10,000 piece 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
Iload,max = 14.5A:
RSENSE
=
-------------V----t-h---,-m----i-n------------- ´ (1 – TF)
1.0A + ILoad, max
=
--------1---0---0---m-----V-----------
1.0A + 14.5A
´
(
1
–
29 % )
=
4.6mW
For discrete resistor and Iload, max = 14.5A:
RSENSE
=
-------------V----t-h---,-m----i-n-------------
1.0A + ILoad, max
´
(1
–
TF)
=
--------1---0---0---m-----V-----------
1.0A + 14.5A
´
(
1
–
5%)
=
6.1mW
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. Rsense for Various Load Current
ILoad,max
(A)
RSENSE
PC Trace
Resistor (mW)
RSENSE
Discrete
Resistor (mW)
10.00
6.5
8.6
11.20
5.8
7.8
12.40
5.3
7.1
13.90
4.8
6.4
14.00
4.7
6.3
14.50
4.6
6.1
14
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