MP2496M – STEP-DOWN CONVERTER WITH SINGLE USB CHARGING PORT

MP2496M Rev.1.0

www.MonolithicPower.com

14

10/10/2015

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APPLICATION INFORMATION

Selecting the Inductor

Use an inductor with a DC current rating at

least 25% higher than the maximum load

current for most applications. Select an inductor

with a small DC resistance for best efficiency.

The inductor value for most designs can be

estimated with Equation (1):

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×Δ ×

(1)

Choose the inductor ripple current to be

approximately 30% of the maximum load

current. The maximum inductor peak current is

calculated with Equation (2):

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

(2)

A 22μH inductance is recommended to improve

EMI.

Selecting the Buck Input Capacitor

The input current to the step-down converter is

discontinuous,

and

therefore

requires

a

capacitor to supply the AC current to the step-

down converter while maintaining the DC input

voltage. For best performance, use low ESR

capacitors. Ceramic capacitors with X5R or

X7R dielectrics are recommended highly

because

of

their

low

ESR

and

small

temperature coefficients. For CLA applications,

a low ESR 100μF electrolytic capacitor and two-

piece

10μF

ceramic

capacitors

are

recommended for EMI reduction.

Since the input capacitor (C1) absorbs the input

switching current, it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated with Equation (3):

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(3)

shown in Equation (4):

2

I

I

LOAD

1

(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 capacitors,

place two high-quality ceramic capacitors as

close to the IC’s IN as possible. The input

voltage ripple caused by the capacitance can

be estimated with Equation (5):

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

(5)

Selecting the Buck Output Capacitor

The device requires an output capacitor (C2) to

maintain the DC output voltage. Estimate the

output voltage ripple with Equation (6):

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

(6)

equivalent series resistance (ESR) value of the

output capacitor.

For tantalum or electrolytic capacitors, ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (7):

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

(7)

The characteristics of the output capacitor

affect the stability of the regulation system. Low

ESR electrolytic capacitors are recommended

for a low output ripple and good control loop

stability. For CLA applications, a 270µF polymer

capacitor or an electrolytic capacitor with

~20mΩ ESR, and one 1µF ceramic capacitor

are recommended.