MPQ2325

– 24V, 3A, SYNCHRONOUS, STEP-DOWN CONVERTER

MPQ2325 Rev. 1.0

www.MonolithicPower.com

12

7/25/2016

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see the Typical Application on

page 1). The feedback resistor (R1) also sets

the feedback loop bandwidth with the internal

compensation capacitor. R2 can be calculated

with Equation (1):

OUT

R1

R2

V

1

0.8V





(1)

The T-type network is highly recommended

(see Figure 5).

Figure 5: T-Type Network

Table 1 lists the recommended T-type resistors

values for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

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

at least 25% higher than the maximum load

current is recommended for most applications.

For the highest efficiency, the inductor DC

resistance should be 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

Choose

the

inductor

current

to

be

approximately 30%

of the maximum load

current. The maximum inductor peak current

can be calculated with Equation (3):

2

I

I

I

L

LOAD

)

MAX

(

L







(3)

Under light-load conditions below 100mA, a

larger inductance is recommended for improved

efficiency.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous

and

therefore

requires

a

capacitor to supply 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 with Equation (4):

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(4)

shown in Equation (5):

2

I

I

LOAD

1

C



(5)

For simplification, choose the 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,

a

small,

high-quality,

ceramic

capacitor (i.e.:

0.1μF) should be placed as close

to the IC as possible. When using ceramic

capacitors, ensure that they have enough

capacitance to provide a sufficient charge to

prevent excessive voltage ripple at the input.

The

input

voltage

ripple

caused

by

the

capacitance can be estimated with Equation (6):

LOAD

OUT

OUT

IN

IN

S

IN

I

V

V

V1

f

C1

V

V









 







(6)