MP21148 – 1A, SYNCHRONOUS, STEP-DOWN CONVERTER WITH CCM

MP21148 Rev. 1.1

www.MonolithicPower.com

11

2/16/2017

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© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see the Typical Application Circuit on

page 15). Select the feedback resistor (R1),

typically between 100k

Ω to 200kΩ, to reduce

requirement on the feedback resistor. An R1

with a value greater than

10kΩ is recommended

for

most

applications.

R2

can

then

be

calculated with Equation (2):

out

R1

R2

V

1

0.6





(2)

Figure 2 shows the feedback circuit.

R1

R2

Vout

FB

MP21148

Figure 2: Feedback Network

Table 1 lists the recommended resistor values

for common output voltages.

Table 1: Resistor Values for Common Output

Voltages

R1 (k

Ω)

R2 (kΩ)

1.0

200 (1%)

300 (1%)

1.2

200 (1%)

200 (1%)

1.8

200 (1%)

100 (1%)

2.5

200 (1%)

63.2 (1%)

3.3

200 (1%)

44.2 (1%)

Selecting the Inductor

Most applications work best with a 0.47µH-to-

2.2µH inductor. Select an inductor with a DC

resistance below 50

mΩ to optimize efficiency.

A high-frequency switch mode power supply

with a magnetic device has a strong, electronic,

magnetic inference for the system. Any un-

shielded power inductor should be avoided.

Metal alloy or multiplayer chip power inductors

are ideal shielded inductors for the application

of the EMI as they can decrease the influence

effectively. Table 2 lists some recommended

inductors.

Table 2: Suggested Inductor List

Manufacturer P/N

Inductance

(μH )

Manufacturer

PIFE25201B-1R0MS

1.0

CYNTEC CO.

LTD.

1239AS-H-1R0M

1.0

Tokyo

74438322010

1.0

Wurth

For most designs, estimate the inductance

value with Equation (3):

OUT

IN

OUT

1

IN

L

OSC

V

(V

V

)

L

V

I

f





  

(3)

Where

∆I

Choose

the

inductor

current

to

be

approximately 30%

of the maximum

load

current. The maximum inductor peak current

can be calculated with Equation (4):

L

L(MAX)

LOAD

I

II

2





(4)

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 10µF capacitor is sufficient. Higher output

voltages may require a 22

μF capacitor to

increase system stability.

Since the input capacitor absorbs the input

switching current, it requires an adequate ripple

current rating. Estimate the RMS current in the

input capacitor with Equation (5):

OUT

OUT

C1

LOAD

IN

IN

VV

I

I

1

VV

















(5)

Equation (6):

LOAD

C1

I

I

2



(6)