REV. D

–12–

ADP3041

As a rule, powdered iron cores saturate softly, whereas ferrite cores

saturate abruptly. Open drum core inductors tend to saturate

gradually and are low cost and small in size, making these

types of inductors attractive in many applications. However,

care must be exercised in their placement because they have

high magnetic fields. In applications that are sensitive to mag-

netic fields, shielded geometrics are recommended.

In addition, inductor losses must be considered. Both core and

copper losses contribute to loss in converter efficiency. To mini-

mize core losses, look for inductors rated for operation at high

switching frequencies. To minimize copper losses, it is best to

use low dc resistance inductors. Typically, it is best to use an

inductor with a dc resistance lower than 20 m

Ω per µH.

The inductor value can be estimated using

LV

V

M

OUT

IN

SLOPE

=

–

(2)

M

f

SLOPE

SW

=

1 456

.

(3)

Choose the closest standard inductor value as a starting point.

The corresponding peak inductor current can then be calculated.

I PEAK

I

V

V

VV

V

LV

f

L

OUT

OUT

IN

IN

OUT

IN

OUT

S









+

×

()

××









1

2

–

(4)

It is recommended to try several different inductor values, sizes,

and types to find the best inductor for the application. In gen-

eral, large inductor values lead to lower ripple current, less

output noise, and either larger size or higher dc resistance.

Conversely, low inductor values lead to higher ripple current,

more noise, and either smaller size or lower dc resistance. The

final inductor selection should be based on the best trade-off of

size, cost, and performance.

Capacitor Selection

The ADP3041 requires an input capacitor to reduce the switching

ripple and noise on the IN pin. The value of the input capaci-

tor depends on the application. For most applications, a minimum

of 10

µF is required. For applications that are running close to

current limit or that have large transient loads, input capacitors

in the range of 22

µF to 47 µF are required.

The selection of the output capacitor also depends on the

the criteria for selecting the output capacitor can be calcu-

lated using

CI

VV

fV

V

OUT

OUT

OUT

IN

S

OUT

OUT

≥×

()

××









8

–

∆

(5)

ESR

V

I PEAK

C

OUT

L

OUT

≤

()

∆

(6)

When selecting an output capacitor, make sure that the ripple

current rating is sufficient to cover the rms switching current

of the ADP3041. The ripple current in the output capacitor is

given by

IC

I

VV

V

RMS

OUT

OUT

OUT

IN

IN

–

(7)

Multilayer ceramic capacitors are a good choice since they have

low ESR, high ripple current rating, and a very small package

size. Tantalum or OS-CON capacitors can be used; however,

they have a larger package size and higher ESR. Table II lists

some capacitor manufacturers. Consult the manufacturer for

more information.

Table II. Capacitor Manufacturers

Vendor

Phone No.

Web Address

AVX

843-448-9411

www.avxcorp.com

Murata

770-436-1300

www.murata.com

Sanyo

408-749-9714

www.sanyovideo.com

Taiyo Yuden

858-554-0755

www.t-yuden.com

Diode Selection

In specifying a diode, consideration must be given to speed,

forward current, forward voltage drop, reverse leakage current,

and the breakdown voltage. The output diode should be rated to

handle the maximum output current. If the output can be sub-

jected to accidental short circuits, then the diode must be rated to

handle currents up to the current limit of the ADP3041. The

breakdown rating of the diode must exceed the output voltage. A

high speed diode with low forward drop and low leakage will help

improve the efficiency of the converter by lowering the losses of

the diode. Schottky diodes are recommended.

Loop Compensation

Like most current programmed PWM converters, the ADP3041

needs compensation to maintain stability over the operating

conditions of the particular application. For operation at duty

cycles above 50%, the choice of inductor is critical in maintain-

ing stability. If the slope of the inductor current is too small or

too large, the circuit will be unstable. See the Inductor Selection

section for more information on choosing the proper inductor.

The ADP3041 provides a pin (COMP) for compensating the

network from the COMP pin to GND (see Figure 2). For most

range of 5 k

selecting the compensation components follow.