MP2172 – 5.5V, 2A, 2.6MHz, SYNCHRONOUS STEP-DOWN SWITCHER

MP2172 Rev. 1.0

www.MonolithicPower.com

14

4/24/2019

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© 2019 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage. The feedback resistor (R1) must

account for both stability and dynamic response

and therefore cannot be too large or too small.

Choose R1 to be around 41.2kΩ. R2 can then

be calculated with Equation (2):

out

R1

R2

V

1

0.6





(2)

The feedback circuit is shown in Figure 2.

R1

VOUT

R2

MP2172

FB

Figure 2: Feedback Network

Table 1 lists the recommended resistor values

for common output voltages.

Table 1: Resistor Values for Common Output

Voltages

VOUT (V)

R1 (kΩ)

R2 (kΩ)

1.0

41.2 (1%)

60.4 (1%)

1.2

41.2 (1%)

41.2 (1%)

1.8

41.2 (1%)

20.5 (1%)

3.3

41.2 (1%)

9.09 (1%)

Selecting the Inductor

A 0.47 - 1.5µH inductor is recommended for

most applications. For the highest efficiency,

choose an inductor with a DC resistance 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)

Where ∆IL is the inductor ripple current.

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. 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 higher output

voltages, use a 47μF capacitor 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)

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

shown in 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 (0.1μF) as close to the IC as possible.

When using ceramic capacitors, ensure that

they have enough capacitance 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

SIN

IV

V

V1

fC1

V

V







 







(7)