MP2159A – 1 A, 6 V, 1.5 MHz SYNCHRONOUS STEP-DOWN CONVERTER

MP2159A Rev. 1.02

www.MonolithicPower.com

12

6/30/2016

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© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see Typical Application on page 1).

The value of 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 120 kΩ to 200 kΩ.

R2 is then given using Equation (2):

out

R1

R2

V

1

0.6





(2)

The feedback circuit is highly recommended

(see Figure 2).

R1

R2

Vout

FB

MP2159A

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 0.68 µH to 2.2 µH inductor is recommended

for most applications. For highest efficiency, the

inductor DC resistance should be less than

15 mΩ. 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 percent of the maximum load

current. The maximum inductor peak current

can be 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, and 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 10 µF capacitor is sufficient. For a higher

output voltage, a 22 μF 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 using 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., 0.1 μF)

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 using

Equation (7):

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V







 







(7)