MP8869 datasheet(31/37 Pages) MPS

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MP8869

– 17V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

PRELIMINARY SPECIFICATIONS SUBJECT TO CHANGE

MP8869 Rev.1.0

www.MonolithicPower.com

6/8/2016

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

Selecting the Inductor

Use a 0.47µH-to-5µH inductor with a DC

current rating 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 5

mΩ.

For most designs, the inductance value can be

derived from Equation (5):

OUT

OSC

)





  

(5)

Where

∆I

L is the inductor ripple current.

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 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 input. The

input voltage ripple caused by 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

MP8869 can be optimized for a wide range of

capacitance and ESR values.

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