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AP1521 Fiches technique(PDF) 5 Page - Anachip Corp

No de pièce AP1521
Description  White LED Step-Up Converter
Download  8 Pages
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Fabricant  ANACHIP [Anachip Corp]
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Logo ANACHIP - Anachip Corp

AP1521 Fiches technique(HTML) 5 Page - Anachip Corp

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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
than the FB pin bias current. For VDC range from
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
current (when the NMOS is off). RSET sets how
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


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