ADP3193AJCPZ-RL View Datasheet(PDF) - Analog Devices
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Description
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ADP3193AJCPZ-RL Datasheet PDF : 32 Pages
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ADP3193A
Solving Equation 5 for an output ripple voltage of 10 mV p-p yields
L ≥ 1.4 V × 1.0 mΩ × (1 − 0.35) = 276 nH
330 kHz × 10 mV
If the resulting ripple voltage is less than what is designed for,
the inductor can be made smaller until the ripple value is met.
This allows optimal transient response and minimum output
decoupling.
The smallest possible inductor should be used to minimize the
number of output capacitors. Choosing a 320 nH inductor is a
good choice for a starting point, and it provides a calculated
ripple current of 11.7 A. The inductor should not saturate at the
peak current of 27.6 A, and it should be able to handle the sum
of the power dissipation caused by the average current of 21.7 A
in the winding and core loss.
Another important factor in the inductor design is the dc resistance
(DCR), which is used for measuring the phase currents. Too large
of a DCR causes excessive power losses, whereas too small of a
value leads to increased measurement error. A good rule is to
have the DCR (RL) be about 1× to 1½× the droop resistance (RO).
This example uses an inductor with a DCR of 1.4 mΩ.
Designing an Inductor
After the inductance and DCR are known, the next step is
either to design an inductor or to find a standard inductor that
best meets the overall design goals. It is also important to have
the inductance and DCR tolerance specified to control the accuracy
of the system. Reasonable tolerances that most manufacturers
can meet are 20% inductance and 7% DCR at room temperature.
The first decision in designing the inductor is choosing the core
material. Several possibilities for providing low core loss at high
frequencies include the powder cores (from Micrometals, Inc., for
example, or Kool-Mu® from Magnetics®) and the gapped soft ferrite
cores (for example, 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 of
inductor, such as a potentiometer core; a PQ, U, or E core; or a
toroid. A good compromise between price and performance is a
core with a toroidal shape.
Many useful magnetics design references are available for
quickly designing a power inductor, such as
• Magnetic Designer Software from Intusoft
• Designing Magnetic Components for High Frequency DC-
DC Converters, by William T. McLyman, K G Magnetics,
Inc., ISBN 1883107008
Selecting a Standard Inductor
The following power inductor manufacturers can provide design
consultation and upon request deliver power inductors optimized
for high power applications.
• Coilcraft, Inc.
• Coiltronics
• Sumida Corporation
CURRENT SENSE AMPLIFIER
Most designs require the regulator output voltage measured at
the CPU pins to droop when the output current increases. The
specified voltage drop corresponds to a dc output resistance (RO),
also referred to as a load line.
The output current is measured by summing the voltage across
each inductor and passing the signal through a low-pass filter. This
summer filter is the CS amplifier configured with Resistor RPH(x)
(summer) and Resistors RCS and CCS (filters). The impedance gain
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)
The user has the flexibility to choose either RCS or RPH(x). However,
it is best to select RCS equal to 100 kΩ, and then solve for RPH(x)
by rearranging Equation 6. In the following example, RO = 1 mΩ
to equal the design load line.
RPH (x )
=
RL
RO
× RCS
1.4 mΩ
RPH(x) = 1.0 mΩ × 100 kΩ = 140 kΩ
Next, use Equation 7 to solve for CCS.
320 nH
CCS = 1.4 mΩ × 100 kΩ = 2.28 nF
It is best to include two locations for CCS in the layout so that
standard values can be used in parallel to better achieve the
desired value. For best accuracy, CCS should be a 5% or 10%
NPO capacitor. This example uses a 5% combination for CCS
of two 1 nF capacitors in parallel. Recalculating RCS and RPH(x)
using this capacitor combination yields 114 kΩ and 160 kΩ.
The closest standard 1% value for RPH(x) is 158 kΩ.
Rev. 0 | Page 20 of 32
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