LTC3413 LINER [Linear Technology], LTC3413 Datasheet - Page 7

• Manufacturer Part Number: LTC3413
• Description: 3A, 2MHz Monolithic Synchronous Regulator for DDR/QDR Memory Termination
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
The basic LTC3413 application circuit is shown in Figure
1a. External component selection is determined by the
maximum load current and begins with the selection of the
inductor value and operating frequency followed by C
and C
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 LTC3413 is determined by
an external resistor that is connected between pin R
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.
Although frequencies as high as 2MHz are possible, the
minimum on-time of the LTC3413 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
decreases with higher inductance.
R
OSC
I
L
OUT
.
V
3 23 10
OUT
.
f
L
•
f
L
increases with higher V
1–
U
11
V
V
OUT
IN
U
–
10
k
W
IN
or V
U
OUT
T
and
and
IN
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.
A reasonable starting point for selecting the ripple current
is I
highest V
below a specified maximum, the inductor value should be
chosen according to the following equation:
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 used
often at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent 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
materials are small and don’t radiate much energy, but
generally cost more than powdered iron core inductors
with similar characteristics. The choice of which style
inductor to use mainly depends on the price versus size
requirements and any radiated field/EMI requirements.
L
L
= 0.4(I
f I
IN
V
. To guarantee that the ripple current stays
L MAX
OUT
MAX
(
). The largest ripple current occurs at the
)
– 1
V
IN MAX
V
OUT
(
)
LTC3413
sn3413 3413fs
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