Revision 12/4/2008

Preliminary and all contents are subject to change without prior notice

Application Hints

LED Current Control

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

is recommended). The formula and table for R1

selection are shown below.

R1 Resistor Value Selection

R1 (

Ω)

5

19.1

10

9.53

12

7.87

15

6.34

20

4.75

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.

Inductor Selection

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.

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.

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

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

LED Driver with Open-Circuit Protection