MP2249 – 6V, 3A, 1MHZ, SYNCHRONOUS STEP-DOWN CONVERTER

MP2249 Rev.1.0

www.MonolithicPower.com

9

11/2/2010

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2010 MPS. All Rights Reserved.

APPLICATION INFORMATION

Output Voltage Setting

The external resistor divider sets the output

voltage (see Page 1, Schematic Diagram). The

feedback resistor R1 also set the feedback loop

bandwidth with the internal compensation.

usually set in the range of 50kHz and 100kHz for

optimal transient performance and good phase

margin. If an electrolytic capacitor is used, the

loop bandwidth is no higher than 1/4 of the ESR

ESR

ESR

O

1

f

2R

C

=

π×

×

150kΩ. R2 is then given by:

OUT

R1

R2

V

1

0.6V

=

−

Table 1—Resistor Selection vs.

Output Voltage Setting

R1

R2

L

1.2V

150kΩ

150kΩ

1.5µH-2.2µH

22µF x 2

1.5V

150kΩ

100kΩ

1.5µH-2.2µH

22µF x 2

1.8V

150kΩ

75kΩ

1.5µH-2.2µH

22µF x 2

2.5V

150kΩ

47.5kΩ

1.5µH-2.2µH

22µF x 2

3.3V

150kΩ

33.2kΩ

1.5µH-2.2µH

22µF x 2

Inductor Selection

A 1.5µH to 2.2µH inductor with DC current rating at

least 25% higher than the maximum load current is

recommended for most applications. For best

efficiency, the inductor DC resistance shall be

<20mΩ. See Table 2 for recommended inductors

and Vendors. For most designs, the inductance

value can be derived from the following equation:

OUT

IN

OUT

IN

L

OSC

Vx(V - V

)

L=

Vx∆Ixf

inductor ripple current approximately 30% of the

maximum load current 3A.

The maximum inductor peak current is:

L

L(MAX)

LOAD

∆I

I= I

+

2

Table 2—Suggested Surface Mount Inductors

Vendor

Part

Number

L

(µH)

DCR

(mΩ)

SC

(A)

L x W x H

(mm

WURTH

744777002

2.2

13

6

7.3×7.3×4.5

744310200

2

14.2

6.5

7×6.9×3

TDK

RLF7030T-

1R5N6R1-T

1.5

8

6.5

7.8×6.8×3.2

Input Capacitor Selection

The input capacitor reduces the surge current

drawn from the input and the switching noise

from the device. The input capacitor impedance

at the switching frequency shall be less than

input

source

impedance

to

prevent

high

frequency switching current passing to the input

source. Ceramic capacitors with X5R or X7R

dielectrics are highly recommended because of

their low ESR and small temperature coefficients.

For most applications, a 47µF capacitor is

sufficient.

Output Capacitor Selection

The output capacitor keeps output voltage ripple

small and ensures a stable regulation loop. The

output capacitor impedance shall be low at the

switching frequency. Ceramic capacitors with

X5R or X7R dielectrics are recommended. If an

electrolytic capacitor is used, pay attention to

output ripple voltage, extra heating, and the

selection of feedback resistor R1 (refer to “Output

Voltage Setting” section) due to the large ESR of

approximately:

OUT

IN

OUT

OUT

IN

OSC

OSC

O

Vx(V - V

)

1

V

x(ESR +

)

Vxf

xL

8xf

xC

∆≤