Fairchild SEMICONDUCTOR RC5040 User Manual
Page 11
APPLICATION NOTE
AN42
11
where:
•
V
IN
= Input Voltage to the Converter
•
V
SW
= Voltage Across the 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
µ
H, and a switching frequency of
650KHz (using C
EXT
= 39pF), the inductor current can be
calculated as follows:
Therefore, for a continued load current of 14.5A, the peak
current through the inductor, I
pk
, is found to be:
For continuous operation at 14.5A, the short circuit detection
threshold must be at least 15.5A.
The next step is to determine the value of the sense resistor.
Including tolerance, the sense resistor value can be approxi-
mated as follows:
where TF = Tolerance Factor for the sense resistor.
Several different types of sense resistors exist. Table 7
describes tolerance, size, power capability, temperature
coefficient and cost of various sense resistors.
I
pk
I
min
–
(
)
2
----------------------------
5.0
14.5
0.037
Ч
–
3.3
–
(
)
1.3
10
6
–
Ч
--------------------------------------------------------------
Ч
=
3.3
0.5
+
(
)
5.0
14.5
0.037
Ч
–
0.5
+
(
)
--------------------------------------------------------------
1
650
10
3
Ч
-----------------------
Ч
1.048A
=
R
SENSE
V
th,min
I
SC
----------------
1
TF
–
(
)
×
V
th,min
1.0
I
Load,max
+
-----------------------------------
1
TF
–
(
)
×
=
=
Table 7. Comparison of Sense Resistors
Refer to Appendix A for Directory of component suppliers
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"
(1 oz Cu trace)
0.45" x 0.065" x
0.200"
0.25" x 0.125" x
0.025"
0.200" x 0.04" x
0.160"
0.200" x 0.04" x
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
@10,000 piece
Low
included in
motherboard
$0.31
$0.47
$0.09
$0.09
I
SC
I
inductor
≥
I
Load, max
I
PK
I
min
–
(
)
2
-----------------------------
+
14.5
1
+
15.5A
=
=
=
Based on the Tolerance in the above table, for embedded PC
trace resistor and for I
load,max
= 14.5A:
For a discrete resistor and I
load, max
= 14.5A:
For user convenience, Table 8 lists the recommended values
for sense resistor at various load currents using an embedded
PC trace resistor or discrete resistor.
R
SENSE
V
th,min
1.0A
I
Load, max
+
-----------------------------------------
1
TF
–
(
)
×
=
=
100mV
1.0A
14.5A
+
---------------------------------
1
29%
–
(
)
×
4.6m
Ω
=
R
SENSE
V
th,min
1.0A
I
Load, max
+
-----------------------------------------
1
TF
–
(
)
×
=
=
100mV
1.0A
14.5A
+
---------------------------------
1
5%
–
(
)
×
6.1m
Ω
=