ADP3050

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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 M

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-

2

DCR. For an 800 mA, 5 V to 3.3 V

system with an inductor DCR of 100 m

Ω, the winding resis-

tance will dissipate (0.8

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 will be between 10 m

Ω 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

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.

2. Calculate the peak switch current (this will be 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 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.

3. Estimate the dc winding resistance based on the inductance

value. A good rule of thumb is to allow about 5 m

Ω of resis-

tance 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 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.

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

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 (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,

and this will determine your maximum ESR (remember

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.

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