MP2136 – 2.5V TO 6V INPUT, 2MHz, 1A SYNCHRONOUS STEP-DOWN CONVERTER

MP2136 Rev. 0.9

www.MonolithicPower.com

9

8/23/2010

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© 2010 MPS. All Rights Reserved.

APPLICATION INFORMATION

Output Voltage Setting

The external resistor divider sets the output

voltage. The feedback resistor R1 also sets the

feedback loop bandwidth with the internal

compensation capacitor.

Choose R1 around 300kΩ for optimal transient

response. R2 is then given by:

1

V

6

.

0

V

1

R

2

R

=

Table 1—Resistor Selection vs. Output

Voltage Setting

R1

R2

1.2V

300k

300k

1.5V

300k

200k

1.8V

300k

150k

2.5V

300k

95.3k

Inductor Selection

A 0.68µH 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. 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

×

∆

×

−

×

=

inductor ripple current approximately 30% of the

maximum load current, 1A.

The maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

∆

+

=

Under light load conditions below 100mA, larger

inductance is recommended for improved

efficiency.

Input Capacitor Selection

The input capacitor reduces the surge current

drawn from the input and 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. Ceramic

capacitors with X5R or X7R dielectrics are

highly recommended because of their low ESR

and small temperature coefficients. The input

capacitor absorbs the input switching current,

so it requires an adequate ripple current rating.

The RMS current through the input capacitor

can be estimated by:

OUT

OUT

CIN

LOAD

IN

IN

VV

II

(1

)

VV

=×

× −

where:

LOAD

CIN

I

I

2

=

For simplification, choose the input capacitor

whose RMS current rating greater than half of

the maximum load current.

For most applications, a 4.7µF capacitor is

sufficient.

Output Capacitor Selection

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.

(

)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

×

×

−

×

≤

∆

3

C

f

8

1

ESR

L

f

V

V

V

V

V

OSC

OSC

IN

OUT

IN

OUT

OUT

For most applications, a 10uF capacitor is

sufficient.