MP2225 –18V/5A SYNCHRONOUS STEP-DOWN CONVERTER

MP2225 Rev. 1.01

www.MonolithicPower.com

14

4/13/2016

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see Typical Application on page 1).

The feedback resistor R1 also sets the

feedback loop bandwidth with the internal

compensation capacitor. First, choose a value

for R1, R2 is then given by:

OUT

R1

R2

V

1

0.6V

=

−

(1)

The feedback network—as shown in Figure 5—

is highly recommended.

Figure 5: Feedback Network

Table 1 lists the recommended resistors and

capacitors value for common output voltages.

Table 1: Component Selection for Common

Output Voltages

(9)

R2 (kΩ)

1.0

120

180

1.2

120

120

1.35

100

80.6

1.5

80.6

53.6

1.8

80.6

40.2

2.5

80.6

25.5

3.3

40.2

8.87

5

40.2

5.49

9) The recommended parameters are based on 500kHz

switching frequency, different output inductors and capacitors

affect the recommended values of R1 and R2. For the other

components’

parameters,

please

refer

to

TYPICAL

APPLICATION CIRCUITS on 17-19.

Selecting the Inductor

Use a 1µH-to-10µH inductor with a DC current

rating at least 25% percent higher than the

maximum load current for most applications.

For highest efficiency, use an inductor with a

DC resistance less than 15mΩ. For most

designs, the inductance value can be derived

from the following equation.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×

−

=

×Δ ×

(2)

Choose the inductor ripple current to be

approximately 30% of the maximum load

current. The maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

(3)

Use a larger inductor for improved efficiency

under light-load conditions—below 100mA.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore requires a capacitor to

supply the AC current to the step-down

converter while maintaining the DC input

voltage. Use low ESR capacitors for the best

performance. Use ceramic capacitors with X5R

or X7R dielectrics for best results because of

their

low

ESR

and

small

temperature

coefficients. For most applications, use a 22µF

capacitor.

Since C1 absorbs the input switching current, it

requires an adequate ripple current rating. The

RMS current in the input capacitor can be

estimated by:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(4)

where:

2

I

I

LOAD

1

(5)

For simplification, choose an input capacitor

with an RMS current rating greater than half of

the maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, add a small, high quality ceramic

capacitor (e.g. 0.1μF) placed as close to the IC