MP2227 – 24V, 3A, 1.3MHz SYNCHRONOUS STEP-DOWN CONVERTER

MP2227 Rev. 1.1

www.MonolithicPower.com

11

11/21/2011

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see Typical Application Schematic). The

feedback resistor R1 also sets the feedback loop

bandwidth

with

the

internal

compensation

capacitor (see Figure 1). The relation between

R1 and feedback loop bandwidth (f

C

capacitance (C

6

CO

1.24 10

R1(K )

f(kHz) C ( F)







.

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

frequency (f

ESR

ESR

O

1

f

2R

C







400KΩ. R2 is then given by:

OUT

R1

R2

V

1

0.8V





Table 1—Resistor Selection vs. Output

Voltage Setting

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

Selecting the Inductor

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

Vx I xf







Where ∆IL is Inductor Ripple Current. Choose

inductor ripple current approximately 30% of the

maximum

load

current,

3A.The

maximum

inductor peak current is:

L

L(MAX)

LOAD

I

II

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 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.

For

most

applications, a 47µF capacitor is sufficient.