LT3592
13
3592fc
at the LT3592 and to force this very high frequency switch-
ing current into a tight local loop, minimizing EMI. A 1µF
capacitor is capable of this task, but only if it is placed
close to the LT3592 and catch diode (see the PCB layout
section). A second precaution regarding the ceramic input
capacitor concerns the maximum input voltage rating of
the LT3592. A ceramic input capacitor combined with trace
or cable inductance forms a high quality (underdamped)
tank circuit. If the LT3592 circuit is plugged into a live
supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3592’s voltage rating.
This situation can easily be avoided, as discussed in the
Hot Plugging Safety section. For more details, see Linear
Technology Application Note 88.
Output Capacitor
For most 2.2MHz LED applications, a 3.3µF or higher
output capacitor is suffi cient for stable operation. A
900kHz application should use a 4.7µF or higher output
capacitor. 400kHz applications require a 22µF or higher
output capacitor. The minimum recommended values
should provide an acceptable (if somewhat underdamped)
transient response, but larger values can always be used
when extra damping is required or desired.
The output capacitor fi lters the inductor current to generate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and stabilizes the LT3592’s
control loop. Because the LT3592 operates at a high fre-
quency, minimal output capacitance is necessary. In addition,
the control loop operates well with or without the presence
of signifi cant output capacitor equivalent series resistance
(ESR). Ceramic capacitors, which achieve very low output
ripple and small circuit size, are therefore an option.
APPLICATIONS INFORMATION
You can estimate output ripple with the following
equation:
V
RIPPLE
=
ΔI
LP−P
8•ƒ•C
OUT
where ΔI
LP-P
is the peak-to-peak ripple current in the in-
ductor. The RMS content of this ripple is very low, so the
RMS current rating of the output capacitor is usually not
a concern. It can be estimated with the formula:
I
C(RMS)
=
ΔI
L
12
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3592 applications. Not all
ceramic capacitors are the same, though. Many of the
higher value ceramic capacitors use poor dielectrics with
high temperature and voltage coeffi cients. In particular,
Y5V and Z5U types lose a large fraction of their capacitance
with applied voltage and at temperature extremes.
Because loop stability and transient response depend on
the value of C
OUT
, this loss may be unacceptable. Use X7R
and X5R types. Table 3 lists several capacitor vendors.
Figure 4 shows the transient response of the LT3592 when
switching between DIM and BRIGHT current levels with
two output capacitor choices. The output load is two series
Luxeon K2 Red LEDs, the DIM current is 50mA and the
BRIGHT current is 500mA, and the circuit is running at
900kHz. The upper photo shows the recommended 4.7µF
value. The second photo shows the improved response
resulting from a larger output capacitor.
Table 3. Capacitor Vendor Information
SUPPLIER PHONE FAX WEBSITE
AVX (803) 448-9411 (803) 448-1943 www.avxcorp.com
Sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com
Taiyo Yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com
TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com
