ADP3031

REV. PrB

–5–

PRELIMINARY TECHNICAL DATA

Table 1. Inductor Manufacturers

Max DC

Max DCR

Height

Part

L(µH)

Current

m

Ω

Ω

Ω

Ω

Ω

(mm)

Vendor

CMD4D11-2R2MC

2.2

0.95

116

1.2

Sumida

CMD4D11-4R7MC

4.7

0.75

216

1.2

847-956-0666

CDRH4D28-100

10

1.00

128

3.0

www.sumida.com

CDRH5D18-220

22

0.80

290

2.0

CR43-4R7

4.7

1.15

109

3.5

CR43-100

10

1.04

182

3.5

DS1608-472

4.7

1.40

60

2.9

Coilcraft

DS1608-103

10

1.00

75

2.9

847-639-6400 www.coilcraft.com

D52LC-4R7M

4.7

1.14

87

2.0

Toko

D52LC-100M

10

0.76

150

2.0

847-297-0070 www.tokoam.com

“grounded” it runs at its highest frequency. A resistor from

RT to ground can be used to set intermediate operating

frequencies.

Because of the large currents which flow in the main

MOSFET switch, it is provided with a separate PGND

return to the negative supply terminal, to avoid corrupting

the small signal return, GND, that can be used as a sense

line at the output load point.

APPLICATION INFORMATION

Frequency Selection

The ADP3031's frequency can be user selected to operate at either

600 KHz or 2 MHz and programmable by setting the RT pin. Tie

RT to GND for 2 MHz operation. For 600 KHz operation, float the

RT pin.

The nominal resistance at the RT pin to get a switching

frequency, f

RT (

Ω) = 320,000 x (2,000,000 - f

2,000,000)

(1)

Output Voltage

The ADP3031 features an adjustable output voltage range of

V

via resistor dividers R1 and R2 (Figure 1.). The feedback

voltage is 1.233 V, so the output voltage is set by the formula:

V

(2)

Since the feedback bias current is 100 nA maximum, R2 may

have a value up to 100 K

Ω with minimum error due to the bias

current.

Inductor Selection

For most of the applications, the inductor used with the

ADP3031 should be in the range of 2 µH to 22 µH. Several

inductor manufacturers are listed in Table 1. When select-

ing an inductor, it is important to make sure that the

inductor used with the ADP3031 is able to handle the peak

current without saturation and that the peak current is

below the current limit of the ADP3031.

As a rule, powdered iron cores saturate softly, whereas

Ferrite cores saturate abruptly. Open drum core inductors

tend to saturate gradually, are low cost and are small in size,

making these types of inductors attractive in many applica-

tions. However, care must be exercised in their placement

because they have high magnetic fields. In applications that

are sensitive to magnetic fields, shielded geometries are

recommended.

In addition, inductor losses must be considered. Both core

and copper losses contribute to loss in converter efficiency.

To minimize 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 the following:

L = (V

Where M

sation.

SLOPE

SW

M

f

1.456

=

Choose the closest standard inductor value as a starting

point.

The corresponding peak inductor current can then be

calculated:

()

()

IN

OUT

IN

OUT

LOUT

IN

OUT

S

VV

V

V

IPEAK

I

VL V

f

1

2

×−





=×

+ 



××





(3)

It is recommended to try several different inductor values,

sizes and types to find the best inductor for the application.

In general, 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.