MP2224

–18V/4A SYNCHRONOUS STEP-DOWN CONVERTER

MP2224 Rev. 1.0

www.MonolithicPower.com

13

12/18/2015

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see Typical Application on page 1).

Also,

the

feedback

resistor

R1

sets

the

feedback loop bandwidth with the internal

compensation capacitor. First, choose a value

for R1, R2 is then given with Equation (1):

OUT

R1

R2

V

1

0.6V





(1)

The feedback network

—as shown in Figure 5—

is highly recommended.

R1

R2

VOUT

FB

8

Figure 5: Feedback Network

Table 1 lists the recommended resistors values

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 a 500kHz

switching frequency. Different output inductors and capacitors

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

components

’ parameters, please refer to the TYPICAL

APPLICATION CIRCUITS on pages 16-18.

Selecting the Inductor

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

rating at least 25% 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 Equation (2).

OUT

IN

OUT

1

IN

L

OSC

V

(V

V

)

L

V

I

f





  

(2)

Where ΔI

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







(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 it 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 with Equation (4):

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(4)

see Equation (5):

2

I

I

LOAD

1

C



(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