MAX1921 Using Ceramic COUT

When using the application circuit of Figure 1, the

inductor’s series resistance causes a small amount of

load regulation, as desired for a voltage-positioning

high at no load by about half of this load regulation. The

simplified calculation is:

inductor. Select a standard resistor value that is within

20% of this calculation.

back node. The simplified calculation is:

where R1 is the standard resistor value that is used.

Select a standard capacitor value that is within 20% of

When using the application circuit of Figure 2, the induc-

tor’s series resistance causes a small amount of load

regulation, as desired for a voltage-positioning load tran-

high at no load by about half of this load regulation:

where R2 is chosen in the 50k

inductor. Use 1% or better resistors.

Next, calculate the equivalent resistance at the FB node as:

fied calculation is:

Select a standard capacitor value that is within 20% of

When using the application circuit of Figure 4, choose

where R2 is chosen to be less than 50k

1.25V. Use 1% or better resistors.

Layout Considerations

High switching frequencies make PC board layout a

very important part of design. Good design minimizes

excessive EMI on the feedback paths and voltage gra-

dients in the ground plane, both of which can result in

instability or regulation errors. Connect the inductor,

input filter capacitor, and output filter capacitor as

close to the device as possible, and keep their traces

short, direct, and wide. Connect their ground pins at a

single common node in a star ground configuration.

The external voltage-feedback network should be very

close to the FB pin, within 0.2in (5mm). Keep noisy

traces, such as the LX trace, away from the voltage-

feedback network; also keep them separate, using

grounded copper. The MAX1920/MAX1921 evaluation

kit data sheet includes a proper PC board layout and

routing scheme.

RR

V

V

OUT

REF

12

1

=×

−









Cq

−

25

10

5

.

Re

Re

||

qR

R

RR

RR

==

×

+

12

12

12

RR

VR

I

MAX

V

OUT

L

OUT

REF

12

2

1

=×

+×

−









() /

CR

−

25

10

1

5

.

RR MAX

L

15

10

4

=×

×

()

Low-Voltage, 400mA Step-Down

DC-DC Converters in SOT23

8

_______________________________________________________________________________________

Table 3. Component Suppliers

SUPPLIER

PHONE

WEBSITE

Coilcraft

847-639-6400

www.coilcraft.com

Kemet

408-986-0424

www.kemet.com

Murata

814-237-1431

www.murata.com

USA

847-956-0666

Sumida

Japan

81-3-3607-5111

www.sumida.com

USA

408-573-4150

www.T-Yuden.com

Taiyo

Yuden

Japan

81-3-3833-5441

www.yuden.co.jp

USA

847-297-0070

www.tokoam.com

Toko

Japan

81-3-3727-1161

www.toko.co.jp

MAX1920

IN

AGND

SHDN

LX

PGND

FB

OFF

ON

R1

L

OUTPUT

UP TO 400mA

INPUT

2V TO 5.5V

R2

2

1

3

5

6

4

Figure 4. MAX1920 Application Circuit Using Tantalum Output

Capacitor