ADP3168 Ver la hoja de datos (PDF) - Analog Devices
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ADP3168 Datasheet PDF : 24 Pages
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DESIGNING AN INDUCTOR
Once the inductance and DCR are known, the next step is to
either 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 control the accuracy of the system. 15% inductance
and 8% DCR (at room temperature) are reasonable tolerances
that most manufacturers can meet.
The first decision in designing the inductor is to choose the
core material. There are several possibilities for providing low
core loss at high frequencies. Two examples are the powder
cores (e.g., Kool-Mµ® from Magnetics, Inc. or Micrometals)
and the gapped soft ferrite cores (e.g., 3F3 or 3F4 from Philips).
Low frequency powdered iron cores should be avoided due to
their high core loss, especially when the inductor value is
relatively low and the ripple current is high.
The best choice for a core geometry is a closed-loop type such
as a pot core, PQ, U, or E core or toroid. A good compromise
between price and performance is a core with a toroidal shape.
There are many useful references for quickly designing a power
inductor, such as the following:
• Magnetic Designer Software
Intusoft (www.intusoft.com)
• Designing Magnetic Components for High-Frequency
DC-DC Converters, by William T. McLyman, Kg
Magnetics, Inc., ISBN 1883107008
SELECTING A STANDARD INDUCTOR
The companies listed below can provide design consultation
and deliver power inductors optimized for high power
applications upon request.
Power Inductor Manufacturers
• Coilcraft
(847)639-6400
www.coilcraft.com
• Coiltronics
(561)752-5000
www.coiltronics.com
• Sumida Electric Company
(510) 668-0660
www.sumida.com
• Vishay Intertechnology
(402) 563-6866
www.vishay.com
ADP3168
OUTPUT DROOP RESISTANCE
The design requires that the regulator output voltage measured
at the CPU pins drops when the output current increases. The
specified voltage drop corresponds to a dc output resistance (RO).
The output current is measured by summing together the
voltage across each inductor and passing the signal through a
low-pass filter. This summer filter is the CS amplifier config-
ured with resistors RPH(X) (summers), and RCS and CCS (filter).
The output resistance of the regulator is set by the following
equations, where RL is the DCR of the output inductors:
RO
=
RCS
RPH (x )
× RL
(6)
CCS
=
RL
L
× RCS
(7)
One has the flexibility of choosing either RCS or RPH(X). It is best
to select RCS equal to 100 kΩ, and then solve for RPH(X) by
rearranging Equation 6.
( ) RPH
x
=
RL
RO
× RCS
RPH (x)
=
1.6
1.3
mΩ
mΩ
× 100
kΩ
=
123
kΩ
Next, use Equation 6 to solve for CCS.
C CS
=
1.6
600 nH
mΩ × 100
kΩ
= 3.75
nF
It is best to have a dual location for CCS in the layout so standard
values can be used in parallel to get as close to the value desired.
For this example, choosing CCS to be 1.5 nF and 2.2 nF in
parallel is a good choice. For best accuracy, CCS should be a 5%
or 10% NPO capacitor. The closest standard 1% value for RPH(X)
is 124 kΩ.
Rev. B | Page 15 of 24
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