MP8869WGL datasheet(34/41 Pages) MPS

34 / 41 page

MP8869W

– 3V - 18V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8869W Rev. 1.02

www.MonolithicPower.com

1/18/2018

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 5mΩ. For most designs, the

inductance value can be derived from Equation

(5):

OUT

OSC

)





  

(5)

Where ∆IL 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 a capacitor

to supply AC current to the step-down converter

while maintaining the DC 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 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,

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

MP8869W can be optimized for a wide range of

capacitance and ESR values.

MP8869WGL Datasheet(PDF) 34 Page - Monolithic Power Systems

MP8869WGL Datasheet(HTML) 34 Page - Monolithic Power Systems

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Datasheet Download

Link URL