AP1521

White LED Step-Up Converter

Anachip Corp.

www.anachip.com.tw

Rev. 0.1 Apr. 27, 2004

4/8

Applications Information

Capacitor Selection

The small size of ceramic capacitors makes them

ideal for AP1521 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

are sufficient for most AP1521 applications.

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).

Diode Selection

Schottky diodes, with their low forward voltage drop

and fast reverse recovery, are the ideal choices for

AP1521 applications. The forward voltage drop of

a Schottky diode represents the conduction losses

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

AP1521.

A Schottky diode rated at 100mA to

200mA is sufficient for most AP1521 applications.

LED Current Control

The LED current is controlled by the feedback

resistor (R1 in Figure 1). The feedback reference

is 310mV.

The LED current is 310mV/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.

(See Table 1)

Table 1. R1 Resistor Value Selection

R1 (Ω)

5

62

10

31

12

25.8

15

20.7

20

15.5

Open-Circuit Protection

In the cases of output open circuit, when the LEDs

are disconnected from the circuit or the LEDs fail,

the feedback voltage will be zero. The AP1521 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 36V rating.

A

zener diode can be used at the output to limit the

voltage on the SW pin (Figure 2).

The zener

voltage should be larger than the maximum forward

voltage of the LED string. The current rating of the

zener should be larger than 0.1mA.

V

IN

SW

SHDN

GND

FB

AP1521

C

OUT

0.22uF

R1

15Ω

L

22uH

D

V

IN

C

IN

1uF

R2

1k

Figure 2. LED Driver with Open-Circuit Protection

Dimming Control

There are four different types of dimming control

circuits:

With the PWM signal applied to the SHDN pin,

the AP1521 is turned on or off by the PWM signal.

The LEDs operate at either zero or full current.

The average LED current increases proportionally

with the duty cycle of the PWM signal. A 0% duty

cycle will turn off the AP1521 and corresponds to

zero LED current. A 100% duty cycle corresponds

to full current. The typical frequency range of the

PWM signal is 1kHz to 10kHz. The magnitude of

the PWM signal should be higher than the minimum

SHDN

voltage.

AP1521

SHDN

PWM