LTC3414
APPLICATIO S I FOR ATIO
The basic LTC3414 application circuit is shown in Figure 1.
External component selection is determined by the maxi-
mum load current and begins with the selection of the
operating frequency and inductor value followed by C
IN
and C
OUT
.
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency of the LTC3414 is determined by
an external resistor that is connected between pin R
T
and
ground. The value of the resistor sets the ramp current that
is used to charge and discharge an internal timing capaci-
tor within the oscillator and can be calculated by using the
following equation:
3.08 • 10
11
R
OSC
=
Ω
– 10k
Ω
f
Although frequencies as high as 4MHz are possible, the
minimum on-time of the LTC3414 imposes a minimum
limit on the operating duty cycle. The minimum on-time is
typically 110ns; therefore, the minimum duty cycle is
equal to 100 • 110ns • f(Hz).
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current
∆I
L
increases with higher V
IN
or V
OUT
and
decreases with higher inductance.
V
V
∆
I
L
=
OUT
1–
OUT
V
IN
f
L
Having a lower ripple current reduces the core losses in
the inductor, the ESR losses in the output capacitors, and
the output voltage ripple. Highest efficiency operation is
achieved at low frequency with small ripple current. This,
however, requires a large inductor.
( )
8
U
A reasonable starting point for selecting the ripple current
is
∆I
L
= 0.4(I
MAX
). The largest ripple current occurs at the
highest V
IN
. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation:
V
OUT
V
OUT
L
=
1–
f
∆
I
L(MAX)
V
IN(MAX)
W
U U
The inductor value will also have an effect on Burst Mode
operation. The transition to 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 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 in-
crease.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. Actual core loss is independent of core size for a
fixed inductor value, but it is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance re-
quires more turns of wire and therefore copper losses will
increase.
Ferrite designs have very low core losses and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don’t radiate much energy, but gener-
ally cost more than powdered iron core inductors with
similar characteristics. The choice of which style inductor
to use mainly depends on the price verus size require-
ments and any radiated field/EMI requirements. New
designs for surface mount inductors are available from
Coiltronics, Coilcraft, Toko, and Sumida.
3414f
LINER [ LINEAR TECHNOLOGY ]
