FAN53168MTC FAIRCHILD [Fairchild Semiconductor], FAN53168MTC Datasheet - Page 17

• Manufacturer Part Number: FAN53168MTC
• Description: 6-Bit VID Controlled 2-4 Phase DC-DC Controller
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

PRODUCT SPECIFICATION
Inductor Selection
The choice of inductance for the inductor determines the
ripple current in the inductor. Less inductance leads to more
ripple current, which increases the output ripple voltage and
conduction losses in the MOSFETs, but allows using
smaller-size inductors and, for a specified peak-to-peak
transient deviation, less total output capacitance. Conversely,
a higher inductance means lower ripple current and reduced
conduction losses, but requires larger-size inductors and
more output capacitance for the same peak-to-peak transient
deviation. In any multi-phase converter, a practical value for
the peak-to-peak inductor ripple current is less than 50% of
the maximum DC current in the same inductor. Equation 4
shows the relationship between the inductance, oscillator
frequency, and peak-to-peak ripple current in the inductor.
Equation 5 can be used to determine the minimum induc-
tance based on a given output ripple voltage:
Solving Equation 5 for a 10 mV
yields:
If the ripple voltage ends up less than that designed for, the
inductor can be made smaller until the ripple value is met.
This will allow optimal transient response and minimum
output decoupling.
The smallest possible inductor should be used to minimize
the number of output capacitors. Choosing a 650nH inductor
is a good choice for a starting point and gives a calculated
ripple current of 8.86A. The inductor should not saturate at
the peak current of 26.1A and should be able to handle the
sum of the power dissipation caused by the average current
of 21.7A in the winding and core loss.
REV. 1.0.0 6/9/03
I
L
R
L
R
DLY
≥
≥
=
V
----------------------------------------------------------------- -
1.5V 1.3mΩ
------------------------------------------------------------------------ -
Ω
VID
V
------------------------------- -
=
O
f
SW
×
×
f
1.96 t
----------------------------------- -
×
228kHz 10mV
SW
(
R
1 D
×
O
×
C
Ω
–
×
L
×
DLY
V
(
DELAY
RIPPLE
)
1
×
×
–
(
(
1 0.375
n D
–
×
)
)
)
p-p
=
534nH
output ripple voltage
Ω
Ω
Ω
Designing an Inductor
Once the inductance and DCR are known, the next step is
either to design an inductor or find a standard inductor that
comes as close as possible to meeting the overall design
goals. It is also important to have the inductance and DCR
tolerance specified to keep the accuracy of the system
controlled. Using 15% for the inductance and 8% for the
DCR (at room temperature) are reasonable tolerances that
most manufacturers can meet.
The best choice for a core geometry is a closed-loop types,
such as pot cores, PQ, U, and E cores, or toroids. A good
compromise between price and performance are cores with a
toroidal shape.
There are many useful references for quickly designing a
power inductor, such as:
Magnetics Design References
1.
2. Designing Magnetic Components for High-Frequency
Selecting a Standard Inductor
The companies listed below can provide design consultation
and deliver power inductors optimized for high power appli-
cations upon request.
Power Inductor Manufacturers
• Coilcraft
• Coiltronics
[phone]
www.coilcraft.com
[phone]
www.coiltronics.com
Magnetic Designer Software
Intusoft (www.intusoft.com)
DC-DC Converters, by William T. McLyman, Kg Mag-
netics, Inc. ISBN 1883107008
Ω
µ
FAN53168
17