MP38115 – 4A, 5.5V SYNCHRONOUS STEP-DOWN SWITCHING REGULATOR

MP38115 Rev. 0.92

www.MonolithicPower.com

6

8/11/2009

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APPLICATION INFORMATION

Output Voltage Setting

The external resistor divider sets the output

voltage (see Page 1, Schematic Diagram). The

feedback resistor R1 also sets the feedback loop

bandwidth with the internal compensation (refer

to description function). The relation between R1

)

F

(

C

)

KHz

(

fc

10

24

.

1

)

K

(

1

R

O

6

μ

×

×

=

Ω

set in the range of 50KHz and 150KHz 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×

×

400KΩ. R2 is then given by:

OUT

R1

R2 =

V

-1

0.8V

Table 1—Resistor Selection vs.

Output Voltage Setting

Vout

R1

R2

L

Cout

(Ceramic)

1.2V

400kΩ

806kΩ

0.47μH-1μH

47μF

1.5V

400kΩ

453kΩ

0.47μH-1μH

47μF

1.8V

400kΩ

316kΩ

0.47μH-1μH

47μF

2.5V

400kΩ

187kΩ

0.47μH-1μH

47μF

3.3V

400kΩ

127kΩ

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

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.

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. The

Δ≤

OUT

IN

OUT

OUT

IN

OSC

OSC

3

Vx(V - V

)

1

V

x(ESR +

)

V xf

xL

8xf

xC

External Schottky Diode

For this part, an external schottky diode is

recommended to be placed close to "SW" and

"GND" pins, especially when the output current is

larger than 2A.

With the external schottky diode, the voltage

spike and negative kick on "SW" pin can be

minimized; moreover, the conversion efficiency

can also be improved a little.

For the external schottky diode selection, it's

noteworthy that the maximum reverse voltage

rating of the external diode should be larger than

the maximum input voltage. As for the current

rating of this diode, 0.5A rating should be

sufficient.