LTC3355 datasheet(13/20 Pages) LINER

13 / 20 page

LTC3355

3355fb

For more information www.linear.com/LTC3355

APPLICATIONS INFORMATION

Buck Inductor L1 Selection and

Maximum Output Current

A good starting point for the inductor value is:

= VOUT + VD

(

)• 1.8

fSW

where fSW is the switching frequency in MHz, VOUT is the

buck output voltage, VD is the catch diode drop (~0.5V)

and L is the inductor value in µH.

The inductor’s RMS current rating must be greater than

the maximum load current and its saturation current

should be 30% higher. To keep the efficiency high, the

series resistance (DCR) should be less than 0.1Ω, and

the core material should be intended for high frequency

applications. Table 1 lists several inductor vendors.

For robust operation and fault conditions (start-up or

short-circuit)andhighinputvoltage(>15V),thesaturation

current should be chosen high enough to ensure that the

inductor peak current does not exceed 2.2A.

The current in the inductor is a triangle wave with an av-

erage value equal to the load current. The peak inductor

and switch current is:

ISW(PEAK) =IL(PEAK) =IOUT(MAX) +

ΔIL

whereIL(PEAK)isthepeakinductorcurrent,IOUT(MAX)isthe

maximumoutputloadcurrentand∆IListheinductorripple

current. The LTC3355 limits the switch current in order to

protect the part. Therefore, the maximum output current

that the buck will deliver depends on the switch current

limit, the inductor value, the input and output voltages.

When the switch is off, the potential across the inductor

is the output voltage plus the catch diode drop. This gives

the peak-to-peak ripple current in the inductor:

ΔIL = 1–DC

(

)• VOUT +VD

L • fSW

where fSW is the switching frequency of the buck, DC is

the duty cycle and L is the value of the inductor.

To maintain output regulation, the inductor peak current

must be less than the buck switch current limit. The

maximum output current is:

IOUT(MAX) =ILIM –

ΔIL

Choosing an inductor value so that the ripple current is

small will allow a maximum output current near the switch

current limit.

Table 1. Inductor Vendors

VENDOR

URL

PART SERIES

TYPE

Murata

www.murata.com

LQH5BPB

Shielded

TDK

www.tdk.com

LTF5022T

Shielded

Toko

www.toko.com

FDS50xx

Shielded

Coilcraft

www.coilcraft.com

XAL40xx, LPS40xx Shielded

Sumida

www.sumida.com

DCRH5D, CDRH6D Shielded

Viashay

www.vishay.com

IHLP2020

Shielded

One approach to choosing the inductor is to start with

the simple rule above, look at the available inductors, and

choose one to meet cost or space goals. Then use the

equations to check that the buck will be able to deliver the

required output current. These equations assume that the

inductor current is continuous. Discontinuous operation

occurs when IOUT is less than ∆IL/2.

LTC3355 Datasheet(PDF) 13 Page - Linear Technology

LTC3355 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL