MP28115 – 4A, 1.5MHz SYNCHRONOUS STEP-DOWN CONVERTER

MP28115 Rev. 0.92

www.MonolithicPower.com

8

8/23/2010

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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 MP2119. In

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

×

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. See Table 2 for recommended

inductors and manufacturers. For most designs,

the inductance value can be derived from the

following equation:

OUT

IN

OUT

IN

L

OSC

Vx(V - V

)

L=

VxΔIxf

where ∆IL is Inductor Ripple Current. Choose

inductor ripple current approximately 30% of the

maximum load current, 4A.

The maximum inductor peak current is:

L

L(MAX)

LOAD

ΔI

I= I

+

2

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.

Table 2—Suggested Surface Mount Inductors

Manufacturer

Part

Number

Inductan

ce

(μH)

Max DCR

(mΩ)

Current Rating

(A)

Dimensions

L x W x H (mm3)

Wurth Electronics

744310055

0.55

4.5

14

7×6.9×3

744310095

0.95

7.4

11

7×6.9×3

TOKO

B1015AS-1R0N

1

11

6.9

8.4×8.3×4