MP8869SGL datasheet(33/39 Pages) MPS

33 / 39 page

MP8869S

– 18V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8869S Rev. 1.03

www.MonolithicPower.com

1/18/2018

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

Choose the inductor ripple current to be

approximately 30% of the maximum load

current. The maximum inductor peak current

can be calculated with Equation (6):

LOAD

)

MAX

(







(6)

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

and

therefore

requires

capacitor to supply AC current to the step-down

converter

while

maintaining

the

input

voltage. Use low ESR capacitors for the best

performance. Ceramic capacitors with X5R or

X7R dielectrics are recommended because of

their

low

ESR

and

small

temperature

coefficients. For most applications, use two

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 with Equation (7):





















OUT

LOAD

(7)

The worst-case condition occurs at VIN =

2VOUT, shown in Equation (8):

LOAD



(8)

For simplification, choose an input capacitor

with an RMS current rating greater than half of

the maximum load current.

The

input

capacitor

can

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, ensure that they have enough

capacitance to provide a sufficient charge to

prevent excessive voltage ripple at the input.

The

input

voltage

ripple

caused

the

capacitance can be estimated with Equation

(9):

LOAD

OUT









 







(9)

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 with Equation (10):

OUT

ESR

8 f







 









 

  (10)

Where L1 is the inductor value, and RESR is the

equivalent series resistance (ESR) value of the

output capacitor.

For

ceramic

capacitors,

the

capacitance

dominates the impedance at the switching

frequency and causes the majority of the output

voltage ripple. For simplification, the output

voltage ripple can be estimated with Equation

(11):

OUT

ΔV

8 f





 





 



(11)

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (12):

OUT

ESR

ΔV





 









(12)

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP8869S can be optimized for a wide range of

capacitance and ESR values.

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