MP28300

– ULTRA-LOW I

MP28300 Rev.1.01

www.MonolithicPower.com

16

6/6/2017

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© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

Inductor Selection

Most applications work best with a 1µH to

2.2µH inductor. Select an inductor with a DC

resistance

less

than

200

mΩ to optimize

efficiency.

High-frequency, switch-mode power supplies

with a magnetic device have strong electronic

magnetic

inference

for

the

system.

Any

unshielded power inductor should be avoided

since it has poor magnetic shielding. Metal alloy

or multiplayer chip power shield inductors are

recommended for the application since they can

decrease influence effectively. Table 2 lists

some recommended inductors.

Table 2: Recommended Inductors

Inductance

Manufacturer

P/N

Package

Manufacturer

2.2µH

DFE201612P-

2R2M

2016

Tokyo

2.2µH

74479775222A

2012

Wurth

For

most

designs,

the

inductance

value can be calculated with Equation (1):

OUT

IN

OUT

1

IN

L

OSC

V

(V

V

)

L

V

I

f





  

(1)

Where

Choose

the

inductor

current

to

be

approximately 30%

of the maximum load

current. The maximum inductor peak current

can be calculated with Equation (2):

L

L(MAX)

LOAD

I

II

2





(2)

Input Capacitor Selection

The input capacitor reduces the surge current

drawn from the input and the switching noise

from the device. Select an input capacitor with a

switching frequency impedance less than the

input

source

impedance

to

prevent

high-

frequency switching current from passing to the

input source. Use low ESR ceramic capacitors

with

X5R

or

X7R

dielectrics

with

small

temperature coefficients. For most applications,

a 10

μF capacitor is sufficient.

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 (3):

OUT

OUT

C1

LOAD

IN

IN

VV

I

I

1

VV

















(3)

shown in Equation (4):

LOAD

C1

I

I

2



(4)

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, add a small, high-quality

, 0.1μF,

ceramic capacitor as close to the IC as possible.

When using ceramic capacitors, ensure that

they have enough capacitance to provide a

sufficient charge to prevent excessive voltage

ripple at the input. The input voltage ripple

caused by capacitance can be estimated with

Equation (5):

LOAD

OUT

OUT

IN

IN

S

IN

I

V

V

V1

f

C1

V

V









 







(5)

Output Capacitor Selection

The output capacitor limits the output voltage

ripple and ensures a stable regulation loop.

Select an output capacitor with low impedance

at

the

switching

frequency.

For

most

applications, a 10µF capacitor is sufficient.

Estimate the output voltage ripple with Equation

(6):

OUT

OUT

OUT

ESR

S

1

IN

S

VV

1

V

1

R

f

L

V

8 f

C2









 











 

 

(6)

equivalent series resistance (ESR) value of the

output

capacitor.

When

using

ceramic

capacitors,

the capacitance dominates the

impedance at the switching frequency and

causes most of the output voltage ripple. For

simplification, the output voltage ripple can be

estimated with Equation (7):

OUT

OUT

OUT

2

S

1

IN

VV

ΔV1

8 f

L

C2

V





 





 



(7)

The characteristics of the output capacitor also

affect the stability of the regulation system.