LTC3607 datasheet(9/20 Pages) LINER

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LTC3607

3607fb

For more information www.linear.com/LTC3607

applicaTions inForMaTion

An important design consideration is that the RDS(ON)

of the P-channel switch increases with decreasing input

supply voltage (see Typical Performance Characteristics).

Therefore, the user should calculate the power dissipation

when the LTC3607 is used at 100% duty cycle with low

input voltage (see Thermal Considerations in the Applica-

tions Information section).

Low/High Supply Operation

TheLTC3607incorporatesanundervoltagelockoutcircuit

which shuts down the part when the input voltage drops

below about 3.7V to prevent unstable operation.

A general LTC3607 application circuit is shown in Figure 1.

Externalcomponentselectionisdrivenbytheloadrequire-

ment, and begins with the selection of the inductor L. Once

the inductor is chosen, CIN and COUT can be selected.

Inductor Selection

The operating frequency directly effects both the inductor

value, and the ripple current. The inductor ripple current

ΔIL decreases with higher frequency and/or inductance

and increases with higher VIN:

∆IL =

VOUT

fO•L

• 1–

VOUT

VIN









Accepting larger values of ΔIL allows the use of low

inductances, but results in higher output voltage ripple,

greater core losses, and lower output current capability.

A reasonable starting point for setting ripple current is

ΔIL = 0.4 • IO(MAX), where IO(MAX) is the maximum rated

output current. The largest ripple current ΔIL occurs at

the maximum input voltage. To guarantee that the ripple

current stays below a specified maximum, the inductor

valueshouldbechosenaccordingtothefollowingequation:

L =

VOUT

fO • IL

• 1–

VOUT

VIN(MAX)









∆

The inductor value will also have an effect on Burst Mode

operation. The transition from low current operation

begins when the peak inductor current falls below a level

set by the burst clamp. Lower inductor values result in

higher ripple current which causes this transition to occur

at lower load currents. This causes a dip in efficiency in

the upper range of low current operation. In Burst Mode

operation, lower inductance values will cause the burst

frequency to increase.

Inductor Core Selection

Different core materials and shapes will change the size/

currentandprice/currentrelationshipofaninductor.Toroid

or shielded pot cores in ferrite or permalloy materials are

small and don’t radiate much energy, but generally cost

more than powdered iron core inductors with similar elec-

trical characteristics. The choice of which style inductor to

use often depends more on the price vs size requirements

and any radiated field/EMI requirements than on what the

LTC3607 requires to operate. Table 1 shows the websites

of several surface mount inductor manufacturers.

Table 1. Inductor Manufacturer

Coilcraft

http://www.coilcraft.com/powersel_lowl.html

Cooper Bussmann http://www.cooperindustries.com/content/public/

en/bussmann/electronics/products/coiltronics_

inductorandtransformermagnetics.html

Würth Electronic

http://katalog.we-online.com/en/pbs/browse/

Power-Magnetics/Speicherdrosseln

Murata

http://www.murata.com/products/inductor/index.

html

TDK

http://www.tdk.co.jp/tefe02/coil.htm

Vishay

http://www.vishay.com/inductors/power-

inductors/

Sumida

http://www.sumida.com/en/products/

power_main.php

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