MP2148 Rev. 1.11

www.MonolithicPower.com

17

5/25/2015

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APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage(Only for

MP2148GQD)

The external resistor divider sets the output

voltage (see the Typical Application on

Figure13). Select the feedback resistor R1 that

consider

reducing

current,

typically between 100kΩ to 200kΩ. There is not

strict requirement on feedback resistor. Select

R1>10kΩ is reasoned for application.

out

R1

R2

V

1

0.6

=

−

Figure 10 shows the feedback circuit.

Figure 10: Feedback Network

Table 1 lists the recommended resistors value

for common output voltages.

Table 1: Resistor Values for Common Output

Voltages

R1 (kΩ)

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

1.5µH inductor. Select an inductor with a DC

resistance

less

than

15mΩ

to

optimize

efficiency.

High frequency switch mode power supply with

magnetic device has strong electronic magnetic

inference for system. Any un-shield power

inductor should be avoided applying as poor

magnetic shielding. Shield inductor, such as

metal alloy or multiplayer chip power are best

candidates for application, can decrease the

influence effectively. Table2 is some suggested

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 from the following equation.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×Δ ×

Choose an inductor current to be approximately

30% of the maximum load current. The

maximum inductor peak current is:

L

L(MAX )

LOAD

I

II

2

Δ

=+

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous,

and

therefore

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. 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 22μF capacitor to

increase system stability.

The input capacitor requires an adequate ripple

current rating because it absorbs the input

switching current. Estimate the RMS current in

the input capacitor with:

OUT

OUT

C1

LOAD

IN

IN

VV

II

1

VV

⎛⎞

⎜⎟

×⎜⎟

⎜⎟

⎝⎠

=×

−

LOAD

C1

I

I

2

=