REV. 0

ADP3170

–9–

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

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

are several possibilities for providing low core loss at high fre-

from 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 inductor 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

20

19

18

17

16

15

14

13

12

11

1

2

3

4

5

6

7

8

9

10

FB

SD

REF

PWRGD

CT

COMP

PGND

DRVH

DRVL

LRDRV

LRFB

VCC

VID3

GND

VID2

VID1

VID0

VID25

CS+

CS–

U1

ADP3170

FROM

CPU

C19

150pF

2.7nF

30.1k

1%

13.7k

1%

C11

1nF

R4

220

R5

220

C8

100pF

C1

1000 F

C2

1000 F

C3

1000 F

C4

1000 F

C5

1000 F

L1

1.7 H

C7

100nF

C6

4.7nF

L1

1 H

Q1

FDB7045L

C12

22 F

Q2

FDB7045L

C13 C14 C15 C16 C17 C18 C19 C20

D1

MBR052LT1

D2

MBR052LT1

5V

12V

5V SB

VTT PWRGD CLK

R1

1k

1.05V – 1.825V

23A

C23

220 F

1.8V SB

1.8V, 200mA

CORE

PWRGD

TO CPU

1000 F

8

RUBYCON ZA SERIES

24m

ESR (EACH)

5V SB

Q6

IRL3103

R6

2.5m

C21

1 F

C22

68pF

R7

10k

Figure 3. 24 A VRM 8.5-Compliant CPU Supply