ADP3050 Data Sheet
Rev. C | Page 12 of 20
For a 400 mA, 24 V to 5 V system
μH7.24
24
5
10200
1
4.02
3
≤×
×
×
×
≤
DIS
L
If the chosen inductor value is too small, the internal current
limit trips each cycle and the regulator has trouble providing the
necessary load current.
Inductor Core Types and Materials
Many types of inductors are currently available. Numerous core
styles along with numerous core materials often make the selection
process seem even more confusing. A quick overview of the
types of inductors available makes the selection process a little
easier to understand.
Open core geometries (bobbin core) are usually less expensive
than closed core geometries (toroidal core) and are 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 generates
electromagnetic 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 are usually smaller than a closed core.
Most core materials used in surface-mount inductors are either
powdered iron or ferrite. For many designs, material choice is
arbitrary, but the properties of each material should be recognized.
Ferrites have lower core losses than powdered iron, but the
lower loss means a higher price. Powdered iron cores saturate
softly (the inductance gradually reduces as current rating is
exceeded), whereas ferrite cores saturate much more abruptly
(the inductance rapidly reduces). Kool Mμ® is one type of ferrite
that is specially designed to minimize core losses and heat
generation (especially at switching frequencies above 100 kHz),
but again, these devices are more expensive.
The winding dc resistance (DCR) of the inductor must not be
overlooked. A high DCR can decrease system efficiency by 2%
to 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
resistance is I
OUT
2
× DCR. For an 800 mA, 5 V to 3.3 V system
with an inductor DCR of 100 mΩ, the winding resistance
dissipates
(0.82 A)
2
× 0.1 Ω = 64 mW. This represents a power loss to the
system of 64 mW/(3.3 V × 800 mA) = 2.4%. Typical DCR
values are 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. Use the following steps to choose an inductor that is
right for the system (refer to the calculations and descriptions
in the Inductor Selection section). Contact the manufacturers
for their full product offering, availability, and pricing. The
manufacturers offer many more values and package sizes to suit
numerous applications.
1. Choose a mode of operation, then calculate the inductor
value using the appropriate equation. For continuous mode
systems, a ripple current of 40% of the maximum load current
is a good starting point. The inductor value can then be
increased or decreased, if desired.
2. Calculate the peak switch current (this is the maximum
current seen by the inductor). Make sure that the dc (or
saturation) current rating of the inductor is high enough
(around 1.2× the peak switch current). Inductors with dc
current ratings of at least 1 A should be used for all
designs. This provides a safety margin for start-up and
fault conditions where the inductor current is higher than
normal. If the current rating of an inductor is exceeded, the
core saturates, causing the inductance value to decrease
and the temperature of the inductor to increase.
3. Estimate the dc winding resistance based on the inductance
value. A general rule is to allow approximately 5 mΩ of
resistance per μH of inductance.
4. Pick the core material and type. First, decide if an open-
core inductor can be used with the design. If this cannot be
determined, try a few samples of each type (open core,
semi closed core, shielded core, and closed core). Do not be
discouraged from using open core inductors because they
require extra care; just be aware of what to look for if used.
They are quite small and inexpensive, and are used
successfully in many different applications.
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 0 in the
feedback loop; therefore, the ESR value must be known so 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
RIPPLE
= ESR × I
RIPPLE
).
Solid tantalum or multilayer ceramic capacitors are 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
manufacturers. Capacitance values from 22 μF to more than 500 μF
can be used, but values of 47 μF to 220 μF are sufficient for most
designs. A smaller value can be used, but ESR is size-dependent,
so a smaller device has a higher ESR. Ensure that the ripple
current of the capacitor rating is larger than the inductor ripple
current (the ripple current flows into the output capacitor).
Multilayer ceramic capacitors can be used in applications where
minimum output voltage ripple is a priority. They have a very
