ap1521wa Diodes, Inc., ap1521wa Datasheet - Page 4

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

White LED Step-Up Converter
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
1
are sufficient for most AP1521 applications.
Inductor Selection
A 22
applications.
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 (C
represents the switching losses.
selection, both forward voltage drop and diode
capacitance need to be considered.
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.
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.
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/I
Table 1. R1 Resistor Value Selection
Anachip Corp.
www.anachip.com.tw
µ
F input capacitor and a 0.22
Applications Information
µ
H inductor is recommended for most AP1521
I
A Schottky diode rated at 100mA to
LED
Not Recommended for New Design
The LED current is 310mV/R1.
LED
10
12
15
20
5
(mA)
Although small size and high
R1 (Ω)
25.8
20.7
15.5
µ
62
31
F output capacitor
(See Table 1)
For diode
Schottky
T
or C
D
In
)
4/8
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.
zener diode can be used at the output to limit the
voltage on the SW pin (Figure 2).
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
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
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.
IN
1uF
C
IN
SHDN
V
IN
22uH
AP1521
GND
L
SW
AP1521
FB
SHDN
PWM
D
SHDN
R2
1k
Rev. 0.1 Apr. 27, 2004
AP1521
Pin
15Ω
R1
The zener
C
0.22uF
OUT
A