MP28251 – 20V, 4A SYNCHRONOUS STEP-DOWN CONVERTER

MP28251 Rev. 1.16

www.MonolithicPower.com

11

4/17/2013

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see Typical Application on page 1).

The feedback resistor, R1, also sets the

feedback loop bandwidth with the internal

compensation capacitor. Choose R1 around

10kΩ, then R2 is:





OUT

R1

R2

V

1

0.815V

The T-type network (shown in Figure 4) is

R8

Figure 4: T-Type Network

Table 1 lists the recommended T-type resistors

value for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

R1 (kΩ)

R2 (kΩ)

R8 (kΩ)

1.05

3.09(1%)

10(1%)

120(1%)

1.2

4.99(1%)

10(1%)

75(1%)

1.8

10(1%)

8.06(1%)

49.9(1%)

2.5

10(1%)

4.75(1%)

24.9(1%)

3.3

10(1%)

3.16(1%)

0(1%)

5

10(1%)

1.91(1%)

0(1%)

Selecting the Inductor

Use a 1µH-to-10µH inductor with a DC current

rating of at least 25% percent higher than the

maximum load current for most applications.

Select an inductor with a DC resistance less

than 15mΩ for best efficiency. Use the following

equation to derive the inductor value for most

designs.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f





 

Choose the inductor ripple current to be

approximately 30% of the maximum load

current. The maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L







Under light-load conditions (below 100mA), use

a larger inductor for improved efficiency.

Setting the Switching Frequency

to

GND

sets

the

MP28251’s

oscillating

calculated from:

)

kHz

(

f

)

k

(

R

S

FREQ

15000





Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous,

and

therefore

requires

a

capacitor to supply the AC current to the step-

down converter while maintaining the DC input

voltage. Use low-ESR capacitors for the best

performance. Ceramic capacitors with X5R or

X7R dielectrics are highly recommended

because

of

their

low

ESR

and

small

temperature coefficients. For most applications,

a 22µF capacitor is sufficient.

Since the input capacitor (C1) absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

where:

2

I

I

LOAD

1