AP1521

White LED Step-Up Converter

Anachip Corp.

www.anachip.com.tw

Rev. 0.1 Apr. 27, 2004

5/8

Applications Information (Continued)

2. Using a DC Voltage

For some applications, the preferred method of

brightness control is a variable DC voltage to adjust

the LED current. The dimming control using a DC

voltage is shown in Figure 3. As the DC voltage

increases, the voltage drop on R2 increases and

the voltage drop on R1 decreases. Thus, the LED

current decreases. The selection of R2 and R3

will make the current from the variable DC source

much smaller than the LED current and much larger

0V to 2V, the selection of resistors in Figure 3 gives

dimming control of LED current from 0mA to 15mA.

3. Using a Filtered PWM Signal

The filtered PWM signal can be considered as an

adjustable DC voltage. It can be used to replace

the variable DC voltage source in dimming control.

The circuit is shown in Figure 4.

AP1521

FB

R1

15Ω

R2

5k

R3

90k

V

DC

Figure 3. Dimming Control Using a DC Voltage

AP1521

FB

R1

15Ω

R2

5k

R3

90k

PWM

10k

0.1uF

Figure 4. Dimming Control Using a Filtered PWM

Signal

4. Using a Logic Signal

For applications that need to adjust the LED current

in discrete steps, a logic signal can be used as

shown in Figure 5. R1 sets the minimum LED

much the LED current increases when the NMOS is

turned on.

Start-up and Inrush Current

To achieve minimum start-up delay, no internal

soft-start circuit is included in AP1521. When first

turned on without an external soft-start circuit,

inrush current is about 200mA.

If soft-start is

desired,

the

recommended

circuit

and

the

waveforms are shown in Figure 6.

If both

soft-start and dimming are used, a 10kHz PWM

signal on SHDN is not recommended. Use a lower

frequency or implement dimming through the FB pin

as shown in Figures 3, 4 or 5.

AP1521

FB

R1

R

SET

Logic

Signal

Figure 5. Dimming Control Using a Logic Signal

AP1521

FB

C

OUT

D1

D2

2.2nF

5k

1k

R2

15Ω

R1

D2:MMBT

Figure 6. Recommended Soft-Startup Circuit