MP9115 – 2.7-6V, 2.5A, 1.2MHZ SYNCHRONOUS STEP-DOWN CONVERTER

MP9115 Rev. 0.92

www.MonolithicPower.com

9

1/14/2010

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2010 MPS. All Rights Reserved.

APPLICATION INFORMATION

Output Voltage Setting

The output voltage is set by the external resistor

divider:

1

OUT

2

R

V0.8 (1

)

R

=⋅ +

The feedforward capacitor is recommended when

output voltage is low, as Figure 2 shows.

Figure 2—Feedback Network

Table 1 lists the recommended resistors and

feedforward capacitors value for common output

voltages.

Table 1—Resistor and Feedforward Capacitor

Selection for Common Output Voltages

Vout

(V)

R1

(kΩ)

R2

(kΩ)

Cf

(pF)

C2 (uF)

Ceramic

L1

(uH)

1.2

120

240

22

44

2.2

1.5

120

137

18

44

2.2

1.8

120

95.3

18

44

2.2

2.5

120

56.2

NS

44

2.2

3.3

120

38.3

NS

44

2.2

Inductor Selection

A 1μH to 10μH 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

<200mΩ.

See

Table2

for

recommended

inductors and manufacturers. For most designs,

the inductance value can be derived from the

following equation:

()

OSC

L

IN

OUT

IN

OUT

f

I

V

V

V

V

L

×

Δ

×

−

×

=

inductor ripple current approximately 30% of the

maximum load current, 2.5A.

The maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Table 2—Suggested Inductors

Manufactu

rer

Part Number

Inductance

(μH)

Dimensions

LxWxH

(mm

Wurth

7447745022

2.2

5.8X5.2X2

TDK

SLF6045T-

2R2N3R3-3PF

2.2

6X6X4.5

The input capacitor reduces the surge current

drawn from the input and switching noise from

the device.

Ceramic capacitors with X5R or X7R dielectrics

are highly recommended because of their low

ESR and small temperature coefficients. For

most applications, a 10μF capacitor is sufficient.

The output capacitor keeps output voltage ripple

small and ensures regulation loop stable. The

output capacitor impedance shall be low at the

switching frequency. Ceramic capacitors with

X5R or X7R dielectrics are recommended. For

forced PWM mode operation, the output ripple

OUT

IN

OUT

OUT

ESR

IN

OSC

OSC

OUT

V(V

V

)

11

V(R

)

Vf

L

8 f

C

⋅

−

Δ=

+ ⋅

⋅⋅

⋅

For most applications, a 33μF capacitor is

sufficient.

Thermal Dissipation

Power dissipation shall be considered when

operates MP9115 at maximum 2.5A output

current. If the junction temperature rises above

150

°C, MP9115 will be shut down by internal

thermal protection circuitry.

The junction-to-ambient thermal resistance of the

maximum

allowable

power

dissipation

is

about1.6W when MP9115 is operating in a 70

°C

ambient temperature environment:

W

6

.

1

W

/

C

50

C

70

C

150

PD

o

o

o

MAX

=

−

=