MP2325

– 24V, 3A SYNC STEP DOWN CONVERTER

MP2325 Rev. 1.0

www.MonolithicPower.com

12

12/4/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see Typical Application on page

1). The feedback resistor R1 also sets the

feedback loop bandwidth with the internal

compensation

capacitor

(see

Typical

Application on page 1). R2 is then given by:

OUT

R1

R2

V

1

0.8V





The T-type network is highly recommended, as

Figure 5 shows.

Figure 5: T-type Network

Table 1 lists the recommended T-type resistors

value for common output voltages.

Table 1

—Resistor Selection for Common

Output Voltages

Cf (µF)

1

20.5

76.8

100

1.8

15

1.2

20.5

39.2

100

1.8

15

1.8

40.2

31.6

56

3.3

15

2.5

40.2

18.7

56

3.3

15

3.3

40.2

12.7

33

4.9

15

5

40.2

7.5

33

4.9

15

Selecting the Inductor

A 1µH to 22µH inductor with a DC current rating

of at least 25% percent higher than the

maximum load current is recommended for

most applications. For highest efficiency, the

inductor DC resistance should be 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

V

I

f





  

Where ΔI

Choose inductor current to be approximately

30%

of

the

maximum

load

current.

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.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required

to supply the AC current to the step-down

converter

while

maintaining

the

DC

input

voltage. Use low ESR capacitors for the best

performance. Ceramic capacitors with X5R or

X7R

dielectrics

are

highly

recommended

because

of

their

low

ESR

and

small

temperature coefficients. For most applications,

a 22µF capacitor is sufficient.

Since the input capacitor (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

The worse case condition occurs at VIN =

2VOUT, where:

2

I

I

LOAD

1

C



For simplification, choose the input capacitor

whose 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,

a

small,

high

quality

ceramic

capacitor, i.e. 0.1μF, should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide sufficient charge to

prevent excessive voltage ripple at input. The

input voltage ripple caused by capacitance can

be estimated by:

LOAD

OUT

OUT

IN

IN

S

IN

I

V

V

V1

f

C1

V

V









 





