LTC1504A
APPLICATIONS INFORMATION
Table 2. Representative Surface Mount Inductors
PART
CoilCraft
DT3316-473
DT3316-104
DO1608-473
DO3316-224
Coiltronics
CTX50-1
CTX100-2
CTX50-1P
CTX100-2P
TP3-470
TP3-470
Sumida
CDRH62-470
CDRH73-101
CD43-470
CD54-101
VALUE
47µH
100µH
47µH
220µH
50µH
100µH
50µH
100µH
47µH
47µH
47µH
100µH
47µH
100µH
MAX DC
1A
0.8A
0.5A
0.8A
0.65A
0.63A
0.66A
0.55A
0.55A
0.72A
0.54A
0.50A
0.54A
0.52A
CORE
TYPE
Shielded
Shielded
Open
Open
Toroid
Toroid
Toroid
Toroid
Toroid
Toroid
Shielded
Shielded
Open
Open
CORE
MATERIAL
Ferrite
Ferrite
Ferrite
Ferrite
KoolMµ
®
KoolMµ
Type 52
Type 52
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
HEIGHT
5.1mm
5.1mm
3.2mm
5.5mm
4.2mm
6mm
4.2mm
6mm
2.2mm
3mm
3mm
3.4mm
3.2mm
4.5mm
Output Capacitor
The output capacitor affects the performance of the
LTC1504A 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 LTC1504A
feedback loop stable. Transient load response of an
LTC1504A circuit is controlled almost entirely by the
output capacitor and the inductor. In steady load opera-
tion, the average current in the inductor will match the load
current. When the load current changes suddenly, the
inductor is suddenly carrying the wrong current and
requires a finite amount of time to correct itself—at least
several switch cycles with typical LTC1504A inductor
values. Even if the LTC1504A 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 cannot 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
Kool Mµ is a registered trademark of Magnetics, Inc..
8
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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 LTC1504A 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
C of the output stage form a 2nd order roll-off with 180°
of phase shift; the R due to ESR forms a single zero at a
somewhat higher frequency that reduces the roll-off to
first order and reduces the phase shift to 90°.
If the output capacitor has a relatively high ESR, the zero
comes in well before the initial phase shift gets all the way
to 180° and the loop only requires a single small capacitor
from COMP to GND to remain stable (Figure 4a). If, on the
other hand, the output capacitor is a low ESR type to
maximize transient response, the ESR zero can increase in
frequency by a decade or more and the output stage phase
shift can get awfully close to 180° before it turns around
and comes back to 90°. Large value ceramic, OS-CON
electrolytic and low impedance tantalum capacitors fall
into this category. These loops require an additional zero
to be inserted at the COMP pin; a series RC in parallel with
a smaller C to ground will usually ensure stability. Figure 4b
shows a typical compensation network which will opti-
mize transient response with most output capacitors.
Adjustable output parts can add a feedforward capacitor
across the feedback resistor divider to further improve
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