MP1495 – SYNCHRONOUS STEP-DOWN CONVERTER

MP1495 Rev. 1.04

www.MonolithicPower.com

12

12/26/2012

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Figure 6: AAM Selection for Common Output

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

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

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

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

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

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

ΔV1

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

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP1495 can be optimized for a wide range of

capacitance and ESR values.