MP9943 – 36V, 3A PEAK SYNCHRONOUS STEP-DOWN CONVERTER

MP9943 Rev.1.1

www.MonolithicPower.com

16

7/4/2017

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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 application, a 22µF ceramic capacitor is

sufficient to maintain the DC input voltage. And it

is strongly recommended to use another lower

value capacitor (e.g. 1µF) with small package

size (0603) to absorb high frequency switching

noise. Make sure place the small size capacitor

as close to IN and GND pins as possible (see

PCB LAYOUT section).

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

where:

2

I

I

LOAD

1

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. 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







 







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







 









 

 

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

∆V1

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

∆V1

R

fL

V



 









The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP9943 can be optimized for a wide range of

capacitance and ESR values.

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP9943 can be optimized for a wide range of

capacitance and ESR values.

BST Resistor and External BST Diode

A 20Ω resistor in series with BST capacitor is

recommended to reduce the SW spike voltage.

Higher resistance is better for SW spike

reduction, but will compromise the efficiency on

the other hand.

BST voltage may become insufficient at some

particular specs. In this case an external

bootstrap diode can enhance the efficiency of the

regulator and avoid BST voltage insufficient at

light load PFM operation. The BST voltage

insufficient is more likely to happen at given

either of following conditions: