KB3440 datasheet(12/20 Pages) KINGBOR

12 / 20 page

The output capacitance is usually many times larger in

order to handle the transient response of the converter.

For a rule of thumb, the ratio of the operating frequency to

the unity-gain bandwidth of the converter is the amount

the output capacitance will have to increase from the

above calculations in order to maintain the desired tran-

sient response.

The other component of ripple is due to the ESR (equiva-

lent series resistance) of the output capacitor. Low ESR

capacitors should be used to minimize output voltage

ripple. For surface mount applications, Taiyo Yuden ce-

ramic capacitors, AVX TPS series tantalum capacitors or

Sanyo POSCAP are recommended.

Input Capacitor Selection

Since the VIN pin is the supply voltage for the IC it is

recommended to place at least a 4.7

µF, low ESR bypass

capacitor.

Table 2. Capacitor Vendor Information

SUPPLIER

PHONE

FAX

WEB SITE

AVX

(803) 448-9411

(803) 448-1943

www.avxcorp.com

Sanyo

(619) 661-6322

(619) 661-1055

www.sanyovideo.com

Taiyo Yuden (408) 573-4150

(408) 573-4159

www.t-yuden.com

Optional Schottky Diodes

To achieve a 1%-2% efficiency improvement above 50mW,

Schottky diodes can be added across synchronous switches

B (SW1 to GND) and D (SW2 to VOUT). The Schottky

diodes will provide a lower voltage drop during the break-

before-make time (typically 15ns) of the NMOS to PMOS

transition. General purpose diodes such as a 1N914 are

not recommended due to the slow recovery times and will

compromise efficiency. If desired a large Schottky diode,

such as an MBRM120T3, can be used from SW2 to VOUT.

A low capacitance Schottky diode is recommended

from GND to SW1 such as a Phillips PMEG2010EA or

equivalent.

Ripple = allowable inductor current ripple

(e.g., 0.2 = 20%)

VIN(MIN) = minimum input voltage, V

VIN(MAX) = maximum input voltage, V

VOUT = output voltage, V

IOUT(MAX) = maximum output load current

For high efficiency, choose an inductor with a high fre-

quency core material, such as ferrite, to reduce core loses.

The inductor should have low ESR (equivalent series

resistance) to reduce the I2R losses, and must be able to

handle the peak inductor current without saturating. Molded

chokes or chip inductors usually do not have enough core

to support the peak inductor currents in the 1A to 2A

region. To minimize radiated noise, use a toroid, pot core

or shielded bobbin inductor. See Table 1 for suggested

components and Table 2 for a list of component suppliers.

Table 1. Inductor Vendor Information

SUPPLIER

PHONE

FAX

WEB SITE

Coilcraft

(847) 639-6400

(847) 639-1469

www.coilcraft.com

Coiltronics

(561) 241-7876

(561) 241-9339

www.coiltronics.com

Murata

USA:

www.murata.com

(814) 237-1431

(814) 238-0490

(800) 831-9172

Sumida

USA:

www.japanlink.com/

(847) 956-0666

(847) 956-0702

sumida

Japan:

81(3) 3607-5111 81(3) 3607-5144

Output Capacitor Selection

The bulk value of the capacitor is set to reduce the ripple

due to charge into the capacitor each cycle. The steady

state ripple due to charge is given by:

•–

•

••

•–

•

••

•

()

(

)

()

Ripple Boost

Ripple Buck

OUT MAX

OUT

IN MIN

OUT

OUT MAX

IN MAX

OUT

IN MAX

OUT

()

100

where COUT = output filter capacitor, F

kB3440

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