REV. 0

ADP3164

–9–

APPLICATION INFORMATION

The design parameters for a typical VRM 9.1-compliant CPU

application are as follows:

Static output voltage drop based on a 0.95 m

load line

1.4605 V – 1.3845 V = 76 mV

Number of Phases (n) = 4

CT Selection—Choosing the Clock Frequency

The ADP3164 uses a fixed-frequency control architecture. The

frequency is set by an external timing capacitor, CT. The clock

frequency determines the switching frequency, which relates

directly to switching losses and the sizes of the inductors and

input and output capacitors. A clock frequency of 800 kHz sets

represents a practical trade-off between the switching losses and

the sizes of the output filter components. To achieve an 800 kHz

oscillator frequency, the required timing capacitor value is 100 pF.

For good frequency stability and initial accuracy, it is recom-

mended to use a capacitor with low temperature coefficient

and tight tolerance, e.g., an MLC capacitor with NPO dielec-

tric and with 5% or less tolerance.

Inductance Selection

The choice of inductance determines the ripple current in the

inductor. Less inductance leads to more ripple current, which

increases the output ripple voltage and the conduction losses in

the MOSFETs, but allows using smaller-size inductors and, for

a specified peak-to-peak transient deviation, output capacitors

with less total capacitance. Conversely, a higher inductance

means lower ripple current and reduced conduction losses, but

requires larger-size inductors and more output capacitance for

the same peak-to-peak transient deviation. In a 4-phase con-

verter, a practical value for the peak-to-peak inductor ripple

current is under 50% of the dc current in the same inductor. A

choice of 50% for this particular design example yields a total

peak-to-peak output ripple current of 8% of the total dc output

current. The following equation shows the relationship between

the inductance, oscillator frequency, peak-to-peak ripple current

in an inductor and input and output voltages.

L

VV

V

Vf

I

IN

OUT

OUT

IN

SW

L RIPPLE

(–

)

()

(1)

For 10 A peak-to-peak ripple current, which is 50% of the

20 A full-load dc current in an inductor, Equation 1 yields an

inductance of:

L

VV

V

V

kHz

A

nH

(– .

)

.

12

1 475

1 475

12

800

4

10

646

A 600 nH inductor can be used, which gives a calculated ripple

current of 10.8 A at no load. The inductor should not saturate

at the peak current of 26 A, and should be able to handle the

sum of the power dissipation caused by the average current of

20 A in the winding and the core loss.

The output ripple current is smaller than the inductor ripple

current due to the four phases partially canceling. This can be

calculated as follows:

I

nV

V

n V

VL

f

I

VV

V

V

nH

kHz

A

O

OUT

IN

OUT

IN

OSC

O

(–

)

.

(–

.

)

.

4 1 475

12

4 1 475

12

600

800

625

(2

Designing an Inductor

Once the inductance is known, the next step is either to design

an inductor or find a standard inductor that comes as close as

possible to meeting the overall design goals. The first decision i

designing the inductor is to choose the core material. There are

several possibilities for providing low core loss at high frequencies.

Two examples are the powder cores (e.g., Kool-M

Magnetics, Inc.) and the gapped soft ferrite cores (e.g., 3F3 or

3F4 from Philips). Low frequency powdered iron cores should

be avoided due to their high core loss, especially when the induc-

tor value is relatively low and the ripple current is high.

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 mag-

netic 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 (http://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 consulta-

tion and deliver power inductors optimized for high power

applications upon request.

Table III. Power Inductor Manufacturers

Coilcraft

(847)639-6400

http://www.coilcraft.com

Coiltronics

(561)752-5000

http://www.coiltronics.com

Sumida Electric Company

(408)982-9660

http://www.sumida.com

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