MP2318 – 24V, 2A SYNC STEP-DOWN CONVERTER

Figure 7: AAM Selection for Common

Output Voltages

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required

to supply the 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

highly

recommended

because

of

their

low

ESR

and

small

temperature coefficients. For most applications,

a 22µF capacitor is sufficient.

Since the input capacitor (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

The worse case condition occurs at VIN =

2VOUT, where:

2

I

I

LOAD

1

C

=

For simplification, choose the input capacitor

whose 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,

a

small,

high

quality

ceramic

capacitor, i.e. 0.1μF, should be 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

S

IN

IV

V

V1

f

C1

V

V



∆

=

×

×−



×



Selecting the Output Capacitor

The output capacitor (C2)

is required to

maintain the DC output voltage. Ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended.

Low

ESR

capacitors

are

preferred to keep the output voltage ripple low.

The output voltage ripple can be estimated by:

OUT

OUT

OUT

ESR

S

1

IN

S

VV

1

V

1R

f

L

V

8 f

C2







∆

=

×−

×

+







×

××

 



equivalent series resistance (ESR) value of the

output capacitor.

In

the

case

of

ceramic

capacitors,

the

impedance

at

the

switching

frequency

is

dominated by the capacitance. The output

voltage

ripple

is

mainly

caused

by

the

capacitance.

For

simplification,

the

output

voltage ripple can be estimated by:

OUT

OUT

OUT

2

IN

S1

VV

ΔV

1

V

8 f

L

C2



=

×−



×

××



In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the

output

ripple

can

be

approximated

to:

MP2318 Rev. 1.0

www.MonolithicPower.com

13

3/2/2014

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