BD9540EFV

Technical Note

10/17

www.rohm.com

2009.04 - Rev.B

© 2009 ROHM Co., Ltd. All rights reserved.

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External Component Selection

1. Inductor (L) selection

※

Passing a current larger than the inductor’s rated current will cause magnetic saturation in the inductor and decrease

system efficiency. When selecting an inductor, be sure to allow enough margin to assure that peak current does not

exceed the inductor’s rated current value.

※

To minimize possible inductor damage and maximize efficiency, choose a inductor with a low (DCR, ACR) resistance.

2. Output Capacitor (CO) Selection

Also, give due consideration to the conditions in formula (7) below for output capacitance, bearing in mind that output rise

time must be established within the soft start time frame:

Note: an improper output capacitor may cause startup malfunctions.

3. Input Capacitor (Cin) Selection

A low-ESR capacitor is recommended to reduce ESR loss and maximize efficiency.

Δ

[A]・・・(3)

L=

Δ

[H]・・・(5)

Δ

Co≦

・・・

(7)

Input Capacitor

[A]・・・(8)

√

2

Δ

IL

VIN

IL

L

Co

VOUT

Output Ripple Current

VIN

L

Co

VOUT

ESR

Output Capacitor

VIN

L

Co

VOUT

Cin

ESL

The inductance value has a major influence on output ripple current.

As formula (3) below indicates, the greater the inductance or

switching frequency, the lower the ripple current.

The proper output ripple current setting is about 30% of maximum

output current.

When determining a proper output capacitor, be sure to factor in the equivalent

series resistance and equivalent series inductance required to set the output ripple

voltage to 20mV or more. Also, make sure the capacitor’s voltage rating is high

enough for the set output voltage (including ripple).

Output ripple voltage is determined as in formula (6) below.

ESL: equivalent series inductance)

Tss: Soft start time

Limit: Over current detection

In order to prevent transient spikes in voltage, the input capacitor selected must

have a low enough ESR resistance to fully support a large ripple current on the

output. The formula for ripple current IRMS is given in equation (8) below:

LG

SW

HG

LG

SW

HG

LG

SW

HG