REV. B

ADP3160/ADP3167

–9–

Two main core types can be used in this application. Open

magnetic loop types, such as beads, beads on leads, and rods and

slugs, provide lower cost but do not have a focused magnetic field

in the core. The radiated EMI from the distributed magnetic

field may create problems with noise interference in the circuitry

surrounding the inductor. Closed-loop types, such as pot cores,

PQ, U, and E cores, or toroids, cost more, but have much

better EMI/RFI performance. A good compromise between

price and performance are cores with a toroidal shape.

There are many useful references for quickly designing a power

inductor. Table II gives some examples.

Table II. Magnetics Design References

Magnetic Designer Software

Intusoft (www.intusoft.com)

Designing Magnetic Components for High-Frequency DC-DC

Converters

McLyman, Kg Magnetics

ISBN 1-883107-00-08

Selecting a Standard Inductor

The companies listed in Table III can provide design consul-

tation and deliver power inductors optimized for high power

applications upon request.

Table III. Power Inductor Manufacturers

Coilcraft

(847) 639-6400

www.coilcraft.com

Coiltronics

(561) 752-5000

www.coiltronics.com

Sumida Electric Company

(510) 668-0660

www.sumida.com

The required equivalent series resistance (ESR) and capacitance

drive the selection of the type and quantity of the output capacitors.

The ESR must be small enough to contain the voltage devia-

tion caused by a maximum allowable CPU transient current

within the specified voltage limits, giving consideration also to the

output ripple and the regulation tolerance. The capacitance must

be large enough that the voltage across the capacitor, which is the

sum of the resistive and capacitive voltage deviations, does not

deviate beyond the initial resistive deviation while the inductor

current ramps up or down to the value corresponding to the new

load current. The maximum allowed ESR also represents the

The cumulative errors in the output voltage regulations cut into

load regulation this relates directly to the ESR. When consider-

ing dc load regulation, this relates directly to the programmed

output resistance of the power converter.

Some error sources, such as initial voltage accuracy and ripple

voltage, can be directly deducted from the available regulation

window. Other error sources scale proportionally to the

amount of voltage positioning used, which, for an optimal design,

should use the maximum that the regulation window will allow.

The error determination is a closed-loop calculation, but it can

be closely approximated. To maintain a conservative design while

avoiding an impractical design, various error sources should

be considered and summed statistically.

The output ripple voltage can be factored into the calculation by

summing the output ripple current with the maximum output

current to determine an effective maximum dynamic current

change. The remaining errors are summed separately according

to the formula:

VV

V

k

I

II

k

k

kk

mV

WIN

VID

VID

O

OO

RCS

CSF

RT

EA

=¥

¥

+

+ Ê

ËÁ

ˆ

¯˜

++

Ê

Ë

Á

Á

ˆ

¯

˜

˜

=

(– (

))

–

D

D

2

1

2

94

2

2

22

(3)

tolerance.

The remaining window is then divided by the maximum output

current plus the ripple to determine the maximum allowed ESR

and output resistance:

RR

V

II

R

mV

AA

m

E MAX

OUT MAX

WIN

OO

E MAX

()

()

()

..

.

==

+

=

+

=

D

W

94

53 4

9 9

15

(4)

The output filter capacitor bank must have an ESR of less than

1.5 m

W. One can, for example, use nine MBZ-type capacitors

from Rubycon, with 2.2 mF capacitance, a 6.3 V voltage rating,

and 13 m

W ESR. The nine capacitors have a maximum total ESR

of 1.44 m

W when connected in parallel. Without ADOPT voltage

positioning, the ESR would need to be less than 0.9 m

W, yielding

a 50% increase to 14 MBZ-type output capacitors.

As long as the capacitance of the output capacitor is above a

critical value and the regulating loop is compensated with

ADOPT, the actual value has no influence on the peak-to-peak

deviation of the output voltage to a full step change in the load

current. The critical capacitance can be calculated as follows:

C

I

RV

L

C

A

m

nH

mF

OUT CRIT

O

E

OUT

OUT CRIT

()

()

.

..

.

=

¥

¥

=

W¥

¥=

2

53 4

144

1 7

600

2

65

(5)

The critical capacitance for the nine Rubycon capacitors with

an equivalent ESR of 1.44 m

W is 6.5 mF, while the equivalent

capacitance of those nine capacitors is 9

¥ 2.2 mF = 19.8 mF.

Therefore, the capacitance is safely above the critical value.