MP8868 – SYNCHRONOUS STEP-DOWN CONVERTER

MP8868 Rev.1.1

www.MonolithicPower.com

22

5/15/2015

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Selecting the Inductor

Use a 0.47µH-to-10µH inductor with a DC

current rating of at least 25% percent higher

than the maximum load current for most

applications. For highest efficiency, use an

inductor with a DC resistance less than 15mΩ.

For most designs, the inductance value can be

derived from the following equation.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×Δ ×

Where ΔIL is the inductor ripple current.

Choose the inductor ripple current to be

approximately 30% of the maximum load

current. The maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Use a larger inductor for improved efficiency

under light-load conditions—below 100mA.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore requires a capacitor is

to supply the AC current to the step-down

converter while maintaining the DC input

voltage. Use low ESR capacitors for the best

performance. Use ceramic capacitors with X5R

or X7R dielectrics for best results because of

their

low

ESR

and

small

temperature

coefficients. For most applications, use two

piece 22µF capacitors.

Since C1 absorbs the input switching current, it

requires an adequate ripple current rating. The

RMS current in the input capacitor can be

estimated by:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

The worse case condition occurs at VIN =

2VOUT, where:

2

I

I

LOAD

1

C

=

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 ceramic

capacitor (e.g. 0.1μF) placed as close to the IC

as possible. When using ceramic capacitors,

make sure that they have enough capacitance

to

provide

sufficient

charge

to

prevent

excessive voltage ripple at input. The input

voltage ripple caused by capacitance can be

estimated by:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

Selecting the Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low-

ESR electrolytic capacitors. For best results,

use low ESR capacitors to keep the output

voltage ripple low. The output voltage ripple can

be estimated by:

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

equivalent series resistance (ESR) value of the

output capacitor.

For

ceramic

capacitors,

the

capacitance

dominates the impedance at the switching

frequency, and the capacitance causes the

majority of the output voltage ripple. For

simplification, the output voltage ripple can be

estimated by:

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated to:

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP8868 can be optimized for a wide range of

capacitance and ESR values.