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LTC3458L

3458lf

COMPONENT SELECTION

Inductor Selection

The high frequency operation of the LTC3458L allows for

the use of small surface mount inductors. Since the

internal slope compensation circuit relies on the inductor’s

current slope and frequency, Table 1 should be used to

select an inductor value for a given frequency of operation

(

± 25%). The recommended value will yield optimal tran-

sient performance while maintaining stable operation.

Inductor values larger than listed in Table 1 are permis-

sible to reduce the current ripple.

Table 1. Recommended Inductor Values

Frequency

Inductor Value(

µH)

1.5MHz

3.3 to 4.7

1.25MHz

4.7 to 6.8

1MHz

6.8 to 10

750kHz

10 to 15

500kHz

15 to 22

For high efficiency, choose an inductor with high fre-

quency core material, such as ferrite, to reduce core

losses. 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 saturat-

ing. Molded chokes or chip inductors usually do not have

enough core to support peak inductor currents in the

1A to 3A region. To minimize radiated noise, use a

toroidal or shielded inductor. (Note that the inductance of

shielded types will drop more as current increases, and

will saturate more easily). See Table 2 for a list of inductor

manufacturers.

Table 2. Inductor Vendor Information

Supplier

Phone

Website

Coilcraft

(847) 639-6400

www.coilcraft.com

TDK

(847) 803-6100

www.component.tdk.com

Murata

USA: (814) 237-1431

(800) 831-9172

www.murata.com

Sumida

USA: (847) 956-0666

Japan: 81-3-3607-5111

www.japanlink.com/sumida

COEV

(800) 227-7040

www.coev.net

Toko

(847) 297-0070

www.tokoam.com

Wurth

(202) 785-8800

www.we-online.com

APPLICATIO S I FOR ATIO

Some example inductor part types are:

Coilcraft: MSS7341 and MSS5131 Series

TDK: RLF5018T and SLF7045 Series

Murata: LQH4C and LQN6C Series

Sumida: CDRH4D28 and CDRH6D28 Series

COEV: DQ7545 Series

TOKO: D62CB and D63LCB Series

WURTH: WE-PD2 Series

Output Capacitor Selection

The output voltage ripple has three components to it. The

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

to charge into the capacitor each cycle. The max ripple due

to charge is given by:

RBULK

OUT

•

••

where IP = peak inductor current and f = switching

frequency.

The ESR (equivalent series resistance) is usually the most

dominant factor for ripple in most power converters. The

ripple due to capacitor ESR is given by:

VRCESR = IP • CESR

where CESR = Capacitor Series Resistance.

The ESL (equivalent series inductance) is also an impor-

tant factor for high frequency converters. Using small,

surface mount ceramic capacitors, placed as close as

possible to the VOUT pins, will minimize ESL.

Low ESR/ESL capacitors should be used to minimize

output voltage ripple. For surface mount applications, AVX

TPS Series tantalum capacitors, Sanyo POSCAP, or Taiyo

Yuden X5R type ceramic capacitors are recommended.

For through-hole applications, Sanyo OS-CON capacitors

offer low ESR in a small package size.

In all applications, a minimum of 4.7

µF (generally 22µF is

recommended), low ESR ceramic capacitor should be

placed as close to the VOUT pin as possible, and grounded

to a local ground plane.

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