LTC3607
applicaTions inForMaTion
An important design consideration is that the R
DS(ON)
VOUT
fO • ∆IL
VOUT
VIN(MAX)
L=
• 1–
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).
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.
Low/High Supply Operation
TheLTC3607incorporatesanundervoltagelockoutcircuit
which shuts down the part when the input voltage drops
below about 3.7V to prevent unstable operation.
AgeneralLTC3607applicationcircuitisshowninFigure1.
Externalcomponentselectionisdrivenbytheloadrequire-
ment,andbeginswiththeselectionoftheinductorL.Once
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-
tricalcharacteristics.Thechoiceofwhichstyleinductorto
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.
the inductor is chosen, C and C
can be selected.
IN
OUT
Inductor Selection
The operating frequency directly effects both the inductor
value, and the ripple current. The inductor ripple current
ΔI decreases with higher frequency and/or inductance
L
and increases with higher V :
IN
V
fO •L
V
VIN
OUT
∆IL = OUT • 1–
Table 1. Inductor Manufacturer
Coilcraft
http://www.coilcraft.com/powersel_lowl.html
Accepting larger values of ΔI allows the use of low
L
Cooper Bussmann http://www.cooperindustries.com/content/public/
en/bussmann/electronics/products/coiltronics_
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
inductorandtransformermagnetics.html
Würth Electronic
Murata
http://katalog.we-online.com/en/pbs/browse/
Power-Magnetics/Speicherdrosseln
ΔI = 0.4 • I
, where I
is the maximum rated
L
O(MAX)
O(MAX)
http://www.murata.com/products/inductor/index.
html
output current. The largest ripple current ΔI occurs at
L
the maximum input voltage. To guarantee that the ripple
current stays below a specified maximum, the inductor
valueshouldbechosenaccordingtothefollowingequation:
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
3607fb
For more information www.linear.com/LTC3607
9
Linear Systems [ Linear Systems ]
