MP2174C

– 5.5V, 4A, 1.1MHZ SYNCHRONOUS STEP-DOWN SWITCHER

MP2174C Rev. 1.0

www.MonolithicPower.com

13

8/7/2019

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2019 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see the Typical Application Circuit

section on page 15). Choose a larger resistance

for lower leakage or a smaller one to avoid noise.

Choose R1 to be between

120kΩ and 200kΩ. R2

is then calculated using Equation (2):





OUT

R1

R2

V

1

0.6

(2)

Figure 2 shows t

he feedback circuit.

Figure 2: Feedback Network

Table 1 lists the recommended resistor values for

common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

VOUT (V)

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 0.82µH to 4.7µH inductor is recommended for

most applications. For highest efficiency, the

inductor DC resistance should be small. For most

designs, the inductance value can be determined

with Equation (3):

OUT

IN

OUT

1

IN

L

OSC

V

(V

V

)

L

V

I

f





  

(3)

Where ΔIL is the inductor ripple current.

Choose the inductor current to be approximately

30% of the maximum load current. Calculated the

maximum inductor peak current with Equation

(4):

2

I

I

I

L

LOAD

)

MAX

(

L







(4)

Selecting the Input Capacitor

The step-down converter has a discontinuous

input current, and a capacitor is required to

supply the AC current to the 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 to stabilize the 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):

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(5)

The worst-case condition occurs at VIN = 2VOUT,

calculated with Equation (6):

2

I

I

LOAD

1

C



(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, place a small, high-quality ceramic

capacitor (e.g.

0.1μF) as close to the IC as

possible. When using ceramic capacitors, ensure

that they have enough capacitance to provide

sufficient charge to prevent excessive voltage

ripple at the input. The input voltage ripple

caused by the capacitance can be estimated with

Equation (7):









 







LOAD

OUT

OUT

IN

IN

SW

IN

I

V

V

V1

f

C1

V

V

(7)

VOUT