MP1601 Rev. 1.0

www.MonolithicPower.com

17

3/24/2016

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APPLICATION INFORMATION

Setting

the

Output

Voltage

(Only

for

MP1601GTF)

The external resistor divider sets the output

voltage (see the Typical Application Circuit on

Figure 12

). Select the feedback resistor (R1),

typically between 40kΩ to 200kΩ, to reduce the

VOUT leakage current. There is no strict

requirement on the feedback resistor. R1 >

10kΩ is reasonable for most applications.

Calculate R2 with Equation (2):

out

R1

R2

V

1

0.6





(2)

Figure 10 shows the feedback circuit.

Figure 10: Feedback Network

Table 1 lists the recommended resistor values

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-

2.2µH inductor. Select an inductor with a DC

resistance below 50mΩ 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, the inductance value can be

calculated with Equation (3):

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f





 

(3)

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 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 Equation (5):

OUT

OUT

C1

LOAD

IN

IN

VV

II

1

VV















(5)

Equation (6):

LOAD

C1

I

I

2



(6)

For simplification, choose an input capacitor

with an RMS current rating greater than half of

the maximum load current.