MP28127 – 1.25A, 5.5V SYNCHRONOUS STEP-DOWN SWITCHING REGULATOR

MP28127 Rev. 0.9

www.MonolithicPower.com

7

6/7/2010

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© 2010 MPS. All Rights Reserved.

APPLICATION INFORMATION

Output Voltage Setting

The external resistor divider sets the output

voltage (see Page 1). The feedback resistor R1

also sets the feedback loop bandwidth with the

internal compensation (refer to description

function). The relation between R1 and feedback

following:

6

CO

1.24 10

R1(K )

f(KHz) C (uF)

×

Ω=

×

than 1/10 of switching frequency of MP28127. In

set to be in the range of 50kHz and 150kHz for

optimal transient performance and good phase

margin. If electrolytic capacitor is used, the loop

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

ESR

ESR

O

1

f

2R

C

=

π×

×

400kΩ. R2 is then given by:

OUT

R1

R2 =

V

-1

0.8V

Table 1—Resistor Selection vs. Output

Voltage Setting

(V)

R1

(kΩ)

R2

(kΩ)

L

(µH)

(ceramic)

1.2

400

806

0.47µH-1µH

47µF

1.5

400

453

0.47µH-1µH

47µF

1.8

400

316

0.47µH-1µH

47µF

2.5

400

187

0.47µH-1µH

47µF

3.3

400

127

0.47µH-1µH

47µF

Inductor Selection

A 0.47µH to 1µH 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 <10mΩ. 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.

The maximum inductor peak current is:

L

L(MAX)

LOAD

∆I

=I

+

2

I

Under light load conditions, larger inductance is

recommended for improved efficiency.

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

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 large ESR of

electrolytic capacitor.

∆≤

OUT

IN

OUT

OUT

IN

OSC

OSC

3

Vx(V - V

)

1

V

x(ESR +

)

Vxf

xL

8xf

xC