Soft-Start

The MAX8640Y/MAX8640Z include internal soft-start

circuitry that eliminates inrush current at startup, reduc-

ing transients on the input source. Soft-start is particu-

larly useful for higher impedance input sources, such

as Li+ and alkaline cells. See the Soft-Start Response

in the

Typical Operating Characteristics.

Applications Information

The MAX8640Y/MAX8640Z are optimized for use with a

tiny inductor and small ceramic capacitors. The correct

selection of external components ensures high efficien-

cy, low output ripple, and fast transient response.

Inductor Selection

A 1µH inductor is recommended for use with the

MAX8640Z, and 2.2µH is recommended for the

MAX8640Y. A 1µH inductor is physically smaller but

requires faster switching, resulting in some efficiency

loss. Table 1 lists several recommended inductors.

It is acceptable to use a 1.5µH inductor with either the

MAX8640Y or MAX8640Z, but efficiency and ripple

should be verified. Similarly, it is acceptable to use a

3.3µH inductor with the MAX8640Y, but performance

should be verified.

For optimum voltage positioning of load transients,

choose an inductor with DC series resistance in the

75m

Ω to 150mΩ range. For higher efficiency at heavy

loads (above 200mA) or minimal load regulation (but

some transient overshoot), the resistance should be

kept as low as possible. For light-load applications up

to 200mA, higher resistance is acceptable with very lit-

tle impact on performance.

Capacitor Selection

Output Capacitor

The output capacitor, C2, is required to keep the output

voltage ripple small and to ensure regulation loop sta-

bility. C2 must have low impedance at the switching fre-

quency. Ceramic capacitors are recommended due to

Tiny 500mA, 4MHz/2MHz Synchronous

Step-Down DC-DC Converters

6

_______________________________________________________________________________________

Table 1. Suggested Inductors

MANUFACTURER

SERIES

INDUCTANCE

(µH)

DC RESISTANCE

(

Ω typ)

CURRENT RATING

(mA)

DIMENSIONS

L x W x H (mm)

MIPFT2520D

2.0

0.16

900

2.5 x 2.0 x 0.5

1.5

0.07

1500

2.2

0.08

1300

FDK

MIPF2520D

3.3

0.10

1200

2.5 x 2.0 x 1.0

1.0

0.12

1200

1.5

0.16

1000

Murata

LQM31P

2.2

0.22

900

3.2 x 1.6 x 0.95

1.2

0.08

590

1.5

0.09

520

Sumida

CDRH2D09

2.2

0.12

440

3.0 x 3.0 x 1.0

1.0

0.11

1100

1.5

0.13

1000

Taiyo Yuden

CKP3216T

2.2

0.14

900

3.2 x 1.6 x 0.9

1.0

0.15

460

GLF201208T

2.2

0.36

300

2.0 x 1.25 x 0.9

1.0

0.07

400

GLF2012T

2.2

0.10

300

2.0 x 1.25 x 1.35

1.0

0.10

800

TDK

GLF251812T

2.2

0.20

600

2.5 x 1.8 x 1.35

1.0

0.05

1000

MDT2520-CR

2.2

0.08

700

2.5 x 2.0 x 1.0

1.0

0.07

1100

TOKO

D2812C

2.2

0.14

770

2.8 x 2.8 x 1.2