MP1495S

– SYNCHRONOUS STEP-DOWN CONVERTER

MP1495S Rev. 1.0

www.MonolithicPower.com

13

12/13/2013

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

© 2013 MPS. All Rights Reserved.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous and therefore requires a capacitor

to supply the AC current to the step-down

converter while maintaining the DC input voltage.

Use

low-ESR

capacitors

for

the

best

performance. For best results, use ceramic

capacitors with X5R or X7R dielectrics because

of

their

low

ESR

and

small

temperature

coefficients. Use a 22µF capacitor for most

applications.

C1 requires an adequate ripple current rating

since it absorbs the input switching current.

Estimate the RMS current in the input capacitor

with:

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

where:

2

I

I

LOAD

1

C



For simplification, choose an 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, place a small, high-quality ceramic

capacitor (e.g.

0.1μF) 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 the input. The input voltage

ripple caused by capacitance can be estimated

by:

LOAD

OUT

OUT

IN

IN

S

IN

I

V

V

V1

f

C1

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

S

1

IN

S

VV

1

V

1

R

f

L

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 thus 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

ΔV

1

V

8 f

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

ΔV

1

R

f

L

V





 









The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP1495S can be optimized for a wide range of

capacitance and ESR values.