constant current. The device can drive two, three or
four LEDs in series from a Li-Ion cell. Series
connection of the LEDs provides identical LED
currents resulting in uniform brightness and
eliminates the need for ballast resistors. The
AP1521 switches at 1MHz that allows the use of
tiny external components. A low 300mV feedback
voltage minimizes power loss in the current setting
resistor for better efficiency.
Applications
- Cellular Phones
- PDAs, Hand-held Computers
- Digital Cameras
- MP3 Players
- GPS Receivers
Pin Assignments
Pin Descriptions
Name
SW
Description
Switch Pin. Connect inductor/diode
here. Minimize trace area at this pin
to reduce EMI.
GND pin
Feedback Pin. Reference voltage is
310mV. Connect cathode of lowest
LED and resistor here. Calculate
resistor value according to the
formula： R
FB
= 310mV / I
LED
Shutdown Pin. Tie to 1.5V or higher
to enable the device 0.4V or less to
；
disable the device.
Input Supply Pin. Must be locally
bypassed.
Top View
V
IN
SHDN
V
SS
FB
SW V
SS
FB
SHDN
SOT23-5L
V
IN
Ordering Information
AP1521 X X
Package
W: SOT23-5L
Packing
Blank : Tube
A : Taping
This datasheet contains new product information. Anachip Corp. reserves the rights to modify the product specification without notice. No liability is assumed as a result of the use of
this product. No rights under any patent accompany the sale of the product.
Rev. 0.1 Apr. 27, 2004
1/8
AP1521
White LED Step-Up Converter
Block Diagram
1 SW
Driver
R
S
Q
Q1
Comparator
-
A2
+
-
FB 3
V
IN
5
V
REF
1.25V
A1
R
C
+
C
C
Σ
RAMP
Generator
-
SHDN 4
Shutdown
1MHz
Oscillator
2 GND
Absolute Maximum Ratings
Symbol
V
IN
V
SW
V
FB
V
SHDN
T
J
T
LEAD
T
OPR
VIN Pin Voltage
SW Voltage
Feedback Pin Voltage
SHDN Pin Voltage
Maximum Junction Temperature
Lead Temperature
Operating Temperature Range
Parameter
Rating
10
36
10
10
125
300
-40 to +85
Unit
V
V
V
V
o
o
o
o
+
310mV
C
C
C
T
STG
Storage Temperature Range
-40 to +125
C
Caution The absolute maximum ratings are rated values exceeding which the product could suffer physical
damage. These values must therefore not be exceeded under any condition.
Anachip Corp.
www.anachip.com.tw
2/8
Rev. 0.1
Apr. 27, 2004
AP1521
White LED Step-Up Converter
Electrical Characteristics
(T
a
=25
°C
, V
IN
= 3V, V
SHDN
= 3V, unless otherwise noted.)
Symbol
V
IN
V
IN
V
FB
I
FB
Parameter
Minimum Operation Voltage
Maximum Operation Voltage
Feedback Pin Voltage
Feedback Pin Bias Current
Supply Current
F
SW
Duty
I
SW
V
SAT
V
SHDN
V
SHDN
I
SHDN
Switching frequency
PWM Maximum Duty Cycle
Switch Current Limit
Switch V
SAT
Switch Leakage Current
SHDN Pin Voltage High
SHDN Pin Voltage Low
SHDN Pin Bias Current
Conditions
-
-
-
-
V
SHDN
= 0V
Min.
2.5
--
280
10
--
--
0.75
85
--
--
--
1.5
--
--
Typ.
--
--
310
45
1.9
0.1
1.0
90
320
350
0.01
--
--
65
Max.
--
10
340
100
2.5
1.0
1.25
--
--
--
5
--
0.4
--
Unit
V
V
mV
nA
mA
µA
MHz
%
mA
mV
µA
V
V
µA
I
SW
= 250mA
V
SW
= 5V
Enable
Disable
Typical Application Circuit
L1
22uH
D1
C2
0.22uF
15mA
LED1
LED2
LED3
R1
20ohm
Vss
FB
SHDN
SW
Vin
C1
1uF
ON OFF
C1, C2: X5R or X7R Dielectric
D1: Central Semiconductor CMDSH-3
L1: MURATA LQH3C-220 or Equivalent
Figure 1.
Anachip Corp.
www.anachip.com.tw
3/8
Rev. 0.1
Apr. 27, 2004
AP1521
White LED Step-Up Converter
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
1
µ
F input capacitor and a 0.22
µ
F output capacitor
are sufficient for most AP1521 applications.
Inductor Selection
A 22
µ
H 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 (C
T
or C
D
)
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/I
LED
(See
Table 1)
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
C
IN
1uF
V
IN
SHDN
GND
SW
R2
1k
R1
15Ω
L
22uH
D
C
OUT
0.22uF
AP1521
FB
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.
Table 1. R1 Resistor Value Selection
I
LED
(mA)
R1 (Ω)
5
62
10
31
12
25.8
15
20.7
20
15.5
AP1521
SHDN
PWM
Anachip Corp.
www.anachip.com.tw
4/8
Rev. 0.1
Apr. 27, 2004
AP1521
White LED Step-Up Converter
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 V
DC
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.
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). R
SET
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
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