LT3474/LT3474-1
APPLICATIONS INFORMATION
A final caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit.Ifpowerisappliedquickly(forexamplebyplugging
the circuit into a live power source), this tank can ring,
doubling the input voltage and damaging the LT3474. The
solution is to either clamp the input voltage or dampen the
tank circuit by adding a lossy capacitor in parallel with the
ceramic capacitor. For details, see Application Note 88.
You can estimate output ripple with the following
equation:
ΔIL
8•f•C
VRIPPLE
=
for ceramic capacitors
OUT
whereΔI isthepeak-to-peakripplecurrentintheinductor.
L
The RMS content of this ripple is very low so the RMS
current rating of the output capacitor is usually not of
concern. It can be estimated with the formula:
Output Capacitor Selection
ΔIL
IC RMS
=
(
)
12
For most LEDs, a 2.2μF 6.3V ceramic capacitor (X5R or
X7R)attheoutputresultsinverylowoutputvoltageripple
and good transient response. Other types and values will
also work; the following discusses tradeoffs in output
ripple and transient performance.
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3474 applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coefficients. In particular, Y5V
and Z5U types lose a large fraction of their capacitance
with applied voltage and at temperature extremes.
Theoutputcapacitorfilterstheinductorcurrenttogenerate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and stabilizes the LT3474’s
control loop. Because the LT3474 operates at a high
frequency, minimal output capacitance is necessary. In
addition, the control loop operates well with or without
the presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
and small circuit size, are therefore an option.
Because loop stability and transient response depend on
the value of C , this loss may be unacceptable. Use X7R
OUT
and X5R types. Table 3 lists several capacitor vendors.
Table 3. Low-ESR Surface Mount Capacitors
VENDOR
Taiyo-Yuden
AVX
TYPE
SERIES
X5R, X7R
X5R, X7R
X5R, X7R
Ceramic
Ceramic
Ceramic
TDK
3474fd
13
Linear [ Linear ]
