lt3475efe-trpbf Linear Technology Corporation, lt3475efe-trpbf Datasheet - Page 12

• Manufacturer Part Number: lt3475efe-trpbf
• Description: Dual Step-down 1.5a Led Driver
• Manufacturer: Linear Technology Corporation

An alternative to a high value ceramic capacitor is a
lower value ceramic along with a larger electrolytic
capacitor. The electrolytic capacitor likely needs to be greater
than 10μF in order to meet the ESR and ripple current
requirements. The input capacitor is likely to see high
surge currents when the input source is applied. Tanta-
lum capacitors can fail due to an over-surge of current.
Only use tantalum capacitors with the appropriate surge
current rating. The manufacturer may also recommend
operation below the rated voltage of the capacitor.
A fi nal 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. If power is applied quickly (for example by plug-
ging the circuit into a live power source) this tank can ring,
doubling the input voltage and damaging the LT3475. 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.
Output Capacitor Selection
For most LEDs, a 2.2μF , 6.3V ceramic capacitor (X5R or
X7R) at the output results in very low output voltage ripple
and good transient response. Other types and values will
also work. The following discusses tradeoffs in output
ripple and transient performance.
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
LT3475’s control loop. Because the LT3475 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.
You can estimate output ripple with the following
equation:
where ΔI
inductor. The RMS content of this ripple is very low so the
LT3475/LT3475-1
12
APPLICATIONS INFORMATION
V
RIPPLE
L
= ΔI
is the peak-to-peak ripple current in the
L
/ (8 • f • C
OUT
) for ceramic capacitors
RMS current rating of the output capacitor is usually not
of concern. It can be estimated with the formula:
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3475 applications. Not all
ceramic capacitors are the same, however. Many of the
higher value 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
and X5R types. Table 3 lists several capacitor vendors.
Table 3. Low ESR Surface Mount Capacitors.
Diode Selection
The catch diode (D3 from the Block Diagram) conducts
current only during switch off time. Average forward cur-
rent in normal operation can be calculated from:
The only reason to consider a diode with a larger current
rating than necessary for nominal operation is for the
worst-case condition of shorted output. The diode cur-
rent will then increase to one half the typical peak switch
current limit.
Peak reverse voltage is equal to the regulator input
voltage. Use a diode with a reverse voltage rating greater
than the input voltage. Table 4 lists several Schottky
diodes and their manufacturers.
Diode reverse leakage can discharge the output capacitor
during LED off times while PWM dimming. If operating at
high ambient temperatures, use a low leakage Schottky
for the widest PWM dimming range.
VENDOR
Taiyo-Yuden
AVX
TDK
I
I
D(AVG)
C(RMS)
= I
= ΔI
OUT
OUT
L
, this loss may be unacceptable. Use X7R
(V
/ 12
IN
– V
Ceramic
Ceramic
Ceramic
OUT
TYPE
)/V
IN
X5R, X7R
X5R, X7R
X5R, X7R
SERIES
3475fb