ap1521wa Diodes, Inc., ap1521wa Datasheet - Page 5

• Manufacturer Part Number: ap1521wa
• Description: White Led Step-up Converter
• Manufacturer: Diodes, Inc.

White LED Step-Up Converter
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.
will make the current from the variable DC source
much smaller than the LED current and much larger
than the FB pin bias current. For V
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.
Figure 3. Dimming Control Using a DC Voltage
Figure 4. Dimming Control Using a Filtered PWM
Signal
Anachip Corp.
www.anachip.com.tw
Applications Information (Continued)
PWM
V
DC
Not Recommended for New Design
10k
R3
90k
AP1521
The selection of R2 and R3
FB
0.1uF
90k
R3
AP1521
R2
5k
FB
R2
5k
15Ω
R1
DC
range from
15Ω
R1
5/8
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.
current (when the NMOS is off). R
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.
desired,
waveforms are shown in Figure 6.
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.
Figure 5. Dimming Control Using a Logic Signal
Figure 6. Recommended Soft-Startup Circuit
D2:MMBT
AP1521
Signal
Logic
the
FB
D1
recommended
2.2nF
D2
R
R1 sets the minimum LED
SET
AP1521
5k
FB
R2
1k
circuit
Rev. 0.1 Apr. 27, 2004
AP1521
15Ω
R1
If soft-start is
SET
R1
and
sets how
C
If both
OUT
the