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PRODUCT SPECIFICATION
RC5040
13
Example:
(7) PD
CAP
I
RMS
2
ESR
′
=
(8) PD
IC
V
CC
I
CC
′
=
DutyCycle
0.5
–
+
0.5
0.3
5
0.73
=
=
PD
MOSFET
10
2
0.030
′
0.73
′
2.19W
=
=
PD
INDUCTOR
10
2
0.010
′
1W
=
=
PD
RSENSE
10
2
0.0065
′
0.65W
=
=
PD
GATE
CV
f
′
5V
′
1.75nf
9
1
–
(
)
V
650Khz
5V
′
′
′
0.045W
=
=
=
PD
DIODE
0.5
10 1
0.73
–
)
′
1.35W
=
=
PD
TRAN
2
5
400pf
′
10
′
650khz
′
0.7A
0.010W
~
=
PD
CAP
7.5
2.5
–
(
)
2
0.015
′
0.37W
=
=
PD
IC
0.2W
=
PD
LOSS
2.19W
1.0W
0.65W
0.045W
1.35W
0.010W
0.37W
0.2W
+
+
+
+
+
+
+
5.815W
=
=
Efficiency
′
10
5.815
+
′
10
3.3
85%
~
=
\
Selecting the Inductor
The inductor is one of the most critical components to be
selected in the DC-DC converter application.. The critical
parameters are inductance (L), maximum DC current (Io)
and the coil resistance (R1). The inductor core material is a
crucial factor in determining the amount of current the
inductor will be able to withstand. As with all engineering
designs, tradeoffs exist between various types of core materi-
als. In general, Ferrites are popular due to their low cost, low
EMI properties and high frequency (>500KHz) characteris-
tics. Molypermalloy powder (MPP) materials exhibit good
saturation characteristics, low EMI and low hysteresis
losses; however, they tend to be expensive and more effec-
tively utilized at operating frequencies below 400KHz.
Another critical parameter is the DC winding resistance of
the inductor. This value should typically be reduced as much
as possible, as the power loss in the DC resistance will
degrade the efficiency of the converter by the relationship:
P
LOSS
= I
O
2
x R1. The value of the inductor is a function of
the oscillator duty cycle (T
ON
) and the maximum inductor
current (I
PK
). I
PK
can be calculated from the relationship:
Where T
ON
is the maximum duty cycle and V
D
is the
forward voltage of diode DS1.
I
PK
I
MIN
V
-------------------L
V
–
V
D
–
è
T
ON
+
=
Then the inductor value can be calculated using the relation-
ship:
Where V
SW
(R
DSON
x I
O
) is the drain-to-source voltage of
M1 when it is switched on.
Implementing Short Circuit Protection
Intel currently requires all power supply manufacturers
to provide continuous protection against short circuit
conditions that may damage the CPU. To address this
requirement, Fairchild Semiconductor has implemented a
current sense methodology to disable the output drive signal
to the MOSFET(s) when an over current condition is
detected. The voltage drop created by the output current
flowing across a sense resistor is presented to one terminal of
an internal comparator with hysterisis. The other comparator
terminal has the threshold voltage, nominally of 120mV.
Table 6 states the limits for the comparator threshold of the
Switching Regulator.
L
V
----------–
V
–
V
O
–
I
PK
I
MIN
è
T
ON
=