Datasheet
8
LTC1504
APPLICATIONS INFORMATION
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half the ripple current and its value should be chosen
based on the desired ripple current and/or the output
current transient requirements. Large value inductors
lower ripple current and decrease the required output
capacitance, but limit the speed that the LTC1504 can
change the output current, limiting output transient re-
sponse. Small value inductors result in higher ripple
currents and increase the demands on the output capaci-
tor, but allow faster output current slew rates and are often
smaller and cheaper for the same DC current rating. A
typical inductor used in an LTC1504 application might
have a maximum current rating between 500mA and 1A
and an inductance between 33µH and 220µH.
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 rod core inductors with similar electrical
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 LTC1504
requires to operate. Table 2 shows some typical surface
mount inductors that work well in LTC1504 applications.
Table 2. Representative Surface Mount Inductors
CORE CORE
PART VALUE MAX DC TYPE MATERIAL HEIGHT
CoilCraft
DT3316-473 47µH 1A Shielded Ferrite 5.1mm
DT3316-104 100µH 0.8A Shielded Ferrite 5.1mm
DO1608-473 47µH 0.5A Open Ferrite 3.2mm
DO3316-224 220µH 0.8A Open Ferrite 5.5mm
Coiltronics
CTX50-1 50µH 0.65A Toroid KoolMµ
®
4.2mm
CTX100-2 100µH 0.63A Toroid KoolMµ 6mm
CTX50-1P 50µH 0.66A Toroid Type 52 4.2mm
CTX100-2P 100µH 0.55A Toroid Type 52 6mm
Sumida
CDRH62-470 47µH 0.54A Shielded Ferrite 3mm
CDRH73-101 100µH 0.50A Shielded Ferrite 3.4mm
CD43-470 47µH 0.54A Open Ferrite 3.2mm
CD54-101 100µH 0.52A Open Ferrite 4.5mm
Output Capacitor
The output capacitor affects the performance of the
LTC1504 in a couple of ways: it provides the first line of
defense during a transient load step and it has a large effect
on the compensation required to keep the LTC1504 feed-
back loop stable. Transient load response of an LTC1504
circuit is controlled almost entirely by the output capacitor
and the inductor. In steady load operation, the average
current in the inductor will match the load current. When
the load current changes suddenly, the inductor is sud-
denly carrying the wrong current and requires a finite
amount of time to correct itself—at least several switch
cycles with typical LTC1504 inductor values. Even if the
LTC1504 had psychic abilities and could instantly assume
the correct duty cycle, the rate of change of current in the
inductor is still related to its value and will not change
instantaneously.
Until the inductor current adjusts to match the load cur-
rent, the output capacitor has to make up the difference.
Applications that require exceptional transient response
(2% or better for instantaneous full-load steps) will re-
quire relatively large value, low ESR output capacitors.
Applications with more moderate transient load require-
ments can often get away with traditional standard ESR
electrolytic capacitors at the output and can use larger
valued inductors to minimize the required output capaci-
tor value. Note that the RMS current in the output capacitor
is slightly more than half of the inductor ripple current—
much smaller than the RMS current in the input bypass
capacitor. Output capacitor lifetime is usually not a factor
in typical LTC1504 applications.
Large value ceramic capacitors used as output bypass
capacitors provide excellent ESR characteristics but can
cause loop compensation difficulties. See the Loop Com-
pensation section.
Loop Compensation
Loop compensation is strongly affected by the output
capacitor. From a loop stability point of view, the output
inductor and capacitor form a series RLC resonant circuit,
with the L set by the inductor value, the C by the value of
the output capacitor and the R dominated by the output
capacitor’s ESR. The amplitude response and phase shift
due to these components is compensated by a network of
Rs and Cs at the COMP pin to (hopefully) close the
feedback loop in a stable manner. Qualitatively, the L and
Kool Mµ is a registered trademark of Magnetics, Inc..