MP2317 – 26V, 1A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP2317 Rev. 1.0

www.MonolithicPower.com

12

3/24/2016

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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 external

compensation capacitor. Calculate R2 with

Equation (1):

OUT

R1

R2

V

1

0.791V





(1)

Table 1 lists the recommended resistor values

for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

3.3

80.6

25.5

10

5

80.6

15

10

Selecting the Inductor

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

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 30mΩ. For

most designs, the inductance value can be

derived from Equation (2):

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f





 

(2)

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. For best performance, use low ESR

capacitors. 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):

OUT

OUT

C1

LOAD

IN

IN

VV

II

1

VV















(4)

shown in Equation (5):

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,

a

small,

high-quality

ceramic

capacitor (i.e.: 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 capacitance

can be estimated with Equation (6):

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V







 







(6)

Selecting the Output Capacitor

The output capacitor (C2) is required to

maintain the DC output voltage. Ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended. For best results, use low ESR

capacitors to keep the output voltage ripple low.

The output voltage ripple can be estimated with

Equation (7):

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2







 









 

 

(7)

equivalent series resistance (ESR) value of the

output capacitor.