ADP3050-3.3 Analog Devices, ADP3050-3.3 Datasheet - Page 9

• Manufacturer Part Number: ADP3050-3.3
• Description: 200 Khz, 1 a Step-down High-voltage Switching Regulator
• Manufacturer: Analog Devices

types of inductors available will make the selection process a
little easier to understand.
Open-core geometries (bobbin core) are usually less expensive than
closed-core geometries (toroidal core) and can be a good choice for
some applications, but care must be taken when they are used. In
open-core inductors, the magnetic flux is not completely contained
inside the core. The radiating magnetic field will generate Electro-
Magnetic Interference (EMI), often inducing voltages onto nearby
circuit board traces. These inductors may not be suitable for systems
that contain very high accuracy circuits or sensitive magnetics. A few
manufacturers have “semiclosed” and “shielded” cores, where an
outer magnetic shield surrounds a bobbin core. These devices have
less EMI than the standard open core, and will usually be smaller
than a closed core.
Most core materials used in surface-mount inductors are either
powdered iron or ferrite. For many designs, material choice will
be arbitrary, but the properties of each material should be recog-
nized. Ferrites will have lower core losses than powdered iron,
but the lower loss means a higher price. Powdered iron cores will
saturate softly (the inductance gradually reduces as current rating
is exceeded), while ferrite cores will saturate much more abruptly
(the inductance rapidly reduces). Kool Mµ
that is specially designed to minimize core losses and heat genera-
tion (especially at switching frequencies above 100 kHz), but again,
these devices will be more expensive.
Do not overlook the dc winding resistance (DCR) of the inductor.
A high DCR can decrease the system efficiency by 2%–5% for
lower output voltages at heavy loads. To obtain a lower DCR
means using a physically larger inductor, so a trade-off in size
and efficiency must be made. The power loss due to this resis-
tance is simply I
system with an inductor DCR of 100 mΩ, the winding resis-
tance will dissipate (0.8
power loss to the system of 64 mW/(3.3 V
Typical DCR values will be between 10 mΩ and 200 mΩ.
Choosing an Inductor
Several considerations must be made when choosing an inductor:
cost, size, EMI, core and copper losses, and maximum current
rating. Follow the steps below to choose an inductor that is right
for the system (refer to the calculations and descriptions from the
previous sections). Table I shows an extensive list of inductors
that can be used. Contact the manufacturers for their full product
offering, availability and pricing. They offer many more values
and package sizes to suit numerous applications.
1. Choose a mode of operation, then calculate the inductor
2. Calculate the peak switch current (this will be the maximum
Kool Mµ is a registered trademark of Magnetics, Inc.
value using the appropriate equation. For continuous mode
systems, a ripple current of 40% the maximum load current
is a good starting point. The inductor value can then be
increased or decreased if desired.
current seen by the inductor). Make sure that the dc (or
saturation) current rating of the inductor is high enough
(around 1.2 times the peak switch current). Inductors with
dc current ratings of at least 1 A should be used for all designs.
This will provide a safety margin for start-up and fault condi-
tions where the inductor current will be higher than normal.
If an inductor’s current rating is exceeded, the core will
saturate, causing the inductance value to decrease and the
inductor’s temperature to increase.
OUT
2
2
DCR. For an 800 mA, 5 V to 3.3 V
0.1 Ω) = 64 mW. This represents a
®
is one type of ferrite
800 mA) = 2.4%.
3. Estimate the dc winding resistance based on the inductance
4. Pick the core material and type. First decide if an open-core
OUTPUT CAPACITOR SELECTION
The ADP3050 can be used with any type of output capacitor.
The trade-offs between price, component size, and regulator
performance can be evaluated to determine the best choice for
each application. The Effective Series Resistance (ESR) of the
capacitor plays an important role in both the loop compensation
and the system performance. The ESR provides a “zero” in the
feedback loop, therefore the ESR value must be known so that
the loop can be compensated correctly (most manufacturers
specify maximum ESR in their data sheets). The capacitor ESR
also contributes to the output ripple voltage (V
I
recommended, providing good performance with a small size
and reasonable cost.
Solid tantalum capacitors have a good combination of low ESR
and high capacitance, and are available from several different manu-
facturers (AVX TPS series, Sprague 593D series, Kemet T495
series, NEMCO LSR series). Capacitance values from 22 µF to more
than 500 µF can be used, but values of 47 µF to 220 µF will be
sufficient for most designs. A smaller value can be used, but ESR is
size-dependent, so a smaller device will have a higher ESR. Ensure
that the capacitor’s ripple current rating is larger than the inductor
ripple current (the ripple current will flow into the output capacitor).
Multilayer ceramic capacitors can be used in applications where
minimum output voltage ripple is a priority. They have a very low
ESR (a 22 µF ceramic can have an ESR one-fifth that of a 22 µF
solid tantalum), but may require more board area for the same
value of output capacitance. A few manufacturers have recently
improved upon their low voltage ceramic capacitors, providing a
smaller package with a lower ESR (Tokin, Murata, Taiyo Yuden
and AVX). Several ceramics can be used in parallel to give an
extremely low ESR and a good value of capacitance. If your design
is very cost-sensitive and not severely space-limited, several alumi-
num electrolytic capacitors can be used in parallel (their size and
ESR are larger than ceramic and solid tantalum). OS-CON
capacitors can also be used, but they are typically larger and
more expensive than ceramic or solid tantalum capacitors.
Choosing an Output Capacitor
Use the following steps to choose an appropriate capacitor.
Several choices for output capacitors are contained in Table III.
1. Decide the maximum output ripple voltage for the design,
RIPPLE
value. A good rule of thumb is to allow about 5 mΩ of resis-
tance per µH of inductance.
inductor can be used with the design. If you are not sure,
you can always get a few samples of each type (open core,
semi-closed core, shielded core, and closed core) and try
them out. Do not be discouraged from using open core
inductors simply because they require a little extra care; just
be aware of what to look for if you do use them. They are
quite small and inexpensive, and are used successfully in
many different applications.
and this will determine your maximum ESR (remember
that V
range between 0.5% and 2% of the output voltage. To lower
the output voltage ripple, there are only two choices: either
increase the inductor value, or use an output capacitor with a
lower ESR.
). Solid tantalum or multilayer ceramic capacitors are
RIPPLE
≈ ESR
I
RIPPLE
). Typical output ripple voltages
ADP3050
RIPPLE
= ESR