MP2158A – 1A, 6V, 1.5MHz SYNCHRONOUS STEP-DOWN CONVERTER

MP2158A Rev. 1.02

www.MonolithicPower.com

11

6/30/2016

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APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see Typical Application on page 1).

The feedback resistor R1 cannot be too large or

too small, considering the trade-off for a

dynamic circuit and stability in the circuit. Set

R1 to around 120kΩ to 200kΩ. R2 is then given

by Equation (2):

out

R1

R2

V

1

0.6





(2)

The feedback circuit is shown in Figure 2.

R1

R2

Vout

FB

MP2158A

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 the highest efficiency,

choose an inductor with a DC resistance of 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)

Set the inductor current to approximately 30%

of the maximum load current. The maximum

inductor peak current is calculated in 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 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 values and small

temperature coefficients. For most applications,

a 10µF capacitor is sufficient. For a higher

output voltage, a 22μF capacitor may be

needed to improve system stability.

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):

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(5)

shown in Equation (6):

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, use a small, high-quality ceramic

capacitor (0.1Μf) and place it as close to the IC

as possible. When using ceramic capacitors,

make sure that they have enough capacitance

to provide a sufficient charge to prevent an

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)