MP8861 – 2.85V - 18V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8861 Rev. 1.1

www.MonolithicPower.com

30

5/28/2020

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The maximum inductor peak current can be

calculated with Equation (6):

2

I

I

I

L

LOAD

)

MAX

(

L







(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):

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(7)

The worst-case condition occurs at VIN =

2VOUT, shown in Equation (8):

2

I

I

LOAD

1

C



(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

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V







 







(9)

Selecting the Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low-

ESR electrolytic capacitors. For the best results,

use low ESR capacitors to keep the output

voltage ripple low. The output voltage ripple can

be estimated with Equation (10):

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2







 









 

  (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

OUT

OUT

2

IN

S1

VV

∆V1

V

8f

L

C2



 



 



(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

OUT

OUT

ESR

IN

S1

VV

∆V1

R

fL

V



 









(12)

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP8861 can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

An external bootstrap diode can enhance the

efficiency of the regulator given the following

conditions:

 VOUT is 5V or 3.3V

 Duty cycle is high: D > 50%

In these cases, add an external BST diode from

VCC to BST (see Figure 13).