MP2161A – 2A, 6V, 1.5MHz SYNCHRONOUS STEP-DOWN CONVERTER

MP2161A Rev. 1.02

www.MonolihicPower.com

12

6/30/2016

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© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

An external resistor divider is used to set the

output voltage (see Typical Application on page

1). The feedback resistor R1 cannot be too

large or too small considering the trade-off

between a dynamic circuit and stability in the

circuit. Choose R1 around 120kΩ to 200kΩ. R2

is then given using Equation (2):

out

R1

R2

V

1

0.6





(2)

The feedback circuit is shown in Figure 2.

R1

R2

Vout

FB

MP2161A

Figure 2: Feedback Network

Table 1 lists the recommended resistor values

for common output voltages.

Table 1—Resistor Selection 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

A 0.68µH to 2.2µH inductor is recommended for

most applications. For highest efficiency, the

inductor DC resistance should be less than

15mΩ. For most designs, the inductance value

can be derived from 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 is

calculated using Equation (4):

2

I

I

I

L

LOAD

)

MAX

(

L







(4)

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required

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. For a higher

output voltage, a 47µF capacitor may be

needed for a more stable system.

Since the input capacitor absorbs the input

switching current, it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated with Equation (5)

and Equation (6):

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(5)

where:

2

I

I

LOAD

1

(6)

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 (e.g., 0.1μF) should be placed as

close to the IC as possible. When using

ceramic capacitors, make sure they have

enough capacitance to provide sufficient charge

to prevent excessive voltage ripple at the input.

The input voltage ripple caused by capacitance

can be estimated with Equation (7):

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V







 







(7)