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AP1521 Fiches technique(PDF) 4 Page - Anachip Corp

No de pièce AP1521
Description  White LED Step-Up Converter
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Fabricant  ANACHIP [Anachip Corp]
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AP1521 Fiches technique(HTML) 4 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
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
1µF input capacitor and a 0.22µF output capacitor
are sufficient for most AP1521 applications.
Inductor Selection
A 22µH inductor is recommended for most AP1521
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
in the diode, while the diode capacitance (CT or CD)
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.
R1 = 310mV/ILED
(See Table 1)
Table 1. R1 Resistor Value Selection
ILED (mA)
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:
1. Using a PWM Signal to SHDN Pin
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


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