Application Hints
Inductor Selection
A
10µH inductor is recommended for most SE3506
applications. Although small size and high efficiency
are major concerns, the inductor should have low
core losses at 1MHz and low DCR (copper wire
resistance). It should also be noted that the
Maximum current rating of the inductor should not
be exceeded in applications. Typically 100mA to
200mA rating is sufficient for most applications.
LED Current Control
The LED
current is controlled by the feedback
resistor (R1). The feedback reference is internally
set at 95mV. The LED current is then determined by
the equation: 95mV/R1. In order to have accurate
LED current, precision resistors are preferred (1%
is recommended). The formula and table for R1
selection are shown below.
R1 = 95mV/I
LED
R1 Resistor Value Selection
I
LED
(mA)
5
10
12
15
20
Capacitor Selection
The small size of ceramic capacitors makes them
ideal for SE3506 applications. X5R and X7R types
are recommended because they retain their
capacitance over wider voltage and temperature
ranges than other types such as Y5V or Z5U. A 1µF
input capacitor and a 1µF output capacitor are
sufficient for most SE3506 applications.
R1 (Ω)
19.1
9.53
7.87
6.34
4.75
Diode Selection
Schottky diodes, with their low forward voltage drop
and fast reverse recovery, are the ideal choices for
SE3506 applications. The forward voltage drop of a
Schottky diode represents the conduction losses in
the diode, while the diode capacitance (C
T
or C
D
)
represents the switching losses. For diode
selection, both forward voltage drop and diode
capacitance need to be considered. Schottky
diodes with higher current ratings usually have
lower forward voltage drop and larger diode
capacitance, which can cause significant switching
losses at the 1MHz switching frequency of the
SE3506. A Schottky diode rated at
100mA to
200mA is sufficient for most SE3506 applications.
Open-Circuit Protection
In the cases of output open circuit, such as when
the LEDs are disconnected from the circuit or the
LEDs fail, the feedback voltage will drop to zero.
The SE3506 will then switch at a high duty cycle
resulting in a high output voltage, which may cause
the SW pin voltage to exceed its maximum 30V
rating. A Zener diode can be used at the output to
limit the voltage on the SW pin. The Zener voltage
should be larger than the maximum forward voltage
of the LED string. For example, a Four-LED string
can be protected by a Zener diode with a
breakdown voltage greater than 16V and less than
30V. The current rating of the zener should be
larger than 10mA.
LED Driver with Open-Circuit Protection
Revision 12/4/2008
Preliminary and all contents are subject to change without prior notice
© Seaward Electronics, Inc., 2006. •
www.seawardinc.com.cn
• Page 3
SEAWARD [ Seaward Electronics Inc. ]
