MP2172C – 2A, SYNCHRONOUS STEP-DOWN SWITCHER

MP2172C Rev.1.0

www.MonolithicPower.com

15

4/22/2019

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

© 2019 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see the Typical Application section on

page 17). Select a feedback resistor (R1) value

to reduce the VOUT leakage current, typically

between 100kΩ and 200kΩ. There is no strict

requirement on the feedback resistor. An R1

value greater than 10kΩ is reasonable for

applications. R2 can then be calculated with

Equation (2):





OUT

R1

R2

V

1

0.6

(2)

Figure 9 shows the feedback circuit.

R1

R2

VOUT

FB

MP2172C

Figure 9: 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

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

Most applications work best with a 1µH to

2.2µH inductor. Select an inductor with a DC

resistance

less

than

50mΩ

to

optimize

efficiency.

A high-frequency, switch-mode power supply

with a magnetic device has strong electronic

magnetic inference (EMI). Any unshielded

power inductors should be avoided. Metal alloy

or multiplayer chip power inductors are ideal

shielded

inductors

since

they

decrease

interference effectively.

Table 2 lists some recommended inductors.

Table 2: Suggested Inductor List

Manufacturer P/N

Inductance

(μH)

Manufacturer

PIFE25201B-

1R0MS

1.0

CYNTEC CO.

LTD.

74437324010

1.0

Wurth

For most designs, estimate the inductance

value with 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 step-down converter has a discontinuous

input current, and requires a capacitor to supply

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 22µF capacitor is sufficient.

Higher output voltages may require a 44μF

capacitor to increase system stability.

The input capacitor requires an adequate ripple

current rating since it absorbs the input

switching current. Estimate the RMS current in

the input capacitor with Equation (5):

OUT

OUT

C1

LOAD

IN

IN

VV

II

1

VV















(5)

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

calculated with Equation (6):

LOAD

C1

I

I

2



(6)