LTC3421 datasheet(11/16 Pages) LINER

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LTC3421

3421f

OPERATIO

disconnect, there must not be any external Schottky

diodes connected between the SW pins and VOUT.

Note: Board layout is extremely critical to minimize volt-

age overshoot on the SW pins due to stray inductance.

Keep the output filter capacitors as close as possible to the

VOUT pins and use very low ESR/ESL ceramic capacitors,

tied to a good ground plane. In VOUT > 4.3V applications,

a Schottky diode is required from the switch nodes to VOUT

to limit the peak switch voltage to less than 6V unless

some form of external snubbing is employed. (See 5V

Applications section.)

APPLICATIO S I FOR ATIO

COMPONENT SELECTION

Inductor Selection

The high frequency operation of the LTC3421 allows the

use of small surface mount inductors. The minimum

inductance value is proportional to the operating fre-

quency and is limited by the following constraints:

and L

f Ripple V

IN MIN

OUT MAX

IN MIN

OUT MAX

()

(

)

()

•–

••

LBI

LBO

VIN

VOUT VOUT

VOUT

SYNC ILIM BURST GND PGND PGND

GND

3421 F01

MULTIPLE VIAS

TO GROUND

PLANE

SHDN

VREF

ENB

VOUTS

VOUT

PGND

VIN

Figure 1. Recommended Component Placement. Traces Carrying

High Current are Direct (PGND, SW, VOUT). Trace Area at FB and

VC are Kept Low. Lead Length to Battery Should be Kept Short.

VIN and VOUT Ceramic Capacitors Should be as Close to the IC

Pins as Possible

where

f = Operating Frequency in MHz

Ripple = Allowable Inductor Current Ripple (Amps

Peak-Peak)

VIN(MIN) = Minimum Input Voltage

VOUT(MAX) = Maximum Output Voltage

The inductor current ripple is typically set to 20% to 40%

of the maximum inductor current.

For high efficiency, choose an inductor with 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 peak inductor currents in the 1A to 4A region.

To minimize radiated noise, use a toroidal or shielded

inductor. See Table 1 for suggested inductor suppliers and

Table 2 for a list of capacitor 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

(516) 241-9339

Murata

USA:

www.murata.com

(814) 237-1431

(814) 238-0490

(800) 831-9172

Sumida

USA:

www.sumida.com

(847) 956-0666

(847) 956-0702

Japan:

81-3-3607-5111 81-3-3607-5144

TDK

(847) 803-6100

(847) 803-6296 www.component.tdk.com

TOKO

(847) 297-0070

(847) 669-7864 www.toko.com

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