LP3303 Datasheet, PDF (6/7 Pages) Lowpower Semiconductor inc – High Performance, Constant Current Switching Regulator For 9PCS White LED

Preliminary Datasheet
Applications Information
LP3303
LED Current Control
The LP3303 regulates the LED current by setting
the current sense resistor (Rset) connecting to
feedback and ground. The internal feedback
reference voltage is 200mV. The LED current can
be set from following equation easily.
ILED(mA)=200mV/Rset
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended). The
table for Rset selection is shown below.
Rset Resistor Value selection
ILED(mA)
RSET(Ω)
20
10
15
13.3
12
16.6
10
20
5
40
Inductor Selection
The recommended value of inductor for 2 to 8
WLEDs applications are 4.7 to 22µH. Small size
and better efficiency are the major concerns for
portable device, such as LP3303 used for mobile
phone. The inductor should have low core loss at
1.4MHz and low DCR for better efficiency. To
avoid inductor saturation current rating should be
considered.
Dimming control
a. Using a PWM Signal to CTRL Pin
For controlling the LED brightness, the LP3303 can
perform the dimming control by applying a PWM
signal to CTRL pin. The internal soft-start and wide
range dimming frequency from 100Hz to 50KHz
can insignificantly reduce audio noise when
dimming. The average LED current is proportional
to the PWM signal duty cycle. The magnitude of the
PWM signal should be higher than the maximum
enable voltage of CTRL pin, in order to let the
dimming control perform correctly.
VIN
22uH
1uF
1uF
6 _VIN
OUT ⁵
PWM
LP3303
4 _CTRL
FB ³
Rset
10R
b. Using a DC Voltage
Using a variable DC voltage to adjust the brightness
is a popular method in some applications. The
dimming control using a DC voltage circuit is
shown in Figure 4. According to the Superposition
Theorem, as the DC voltage increases, the voltage
contributed to VFB increases and the voltage drop
on RSET decreases, i.e. the LED current decreases.
For example, if the VDC range is from 0V to 2.8V,
the selection of resistors in Figure 4 sets dimming
control of LED current from 20mA to 0mA.
VI N
22uH
1uF
1uF
6
VI N
5
OUT
LP3303
ON
OFF
4
CTRL
3
FB
R1
5K
R2
90K
VDC
(0-2.8V)
Rset
10R
c. Using a Filtered PWM signal
Another common application is using a filtered
PWM signal as an adjustable DC voltage for LED
dimming control. A filtered PWM signal acts as the
DC voltage to regulate the output current. The
recommended application circuit is shown in the
Figure 6. In this circuit, the output ripple depends
on the frequency of PWM signal. For smaller output
voltage ripple (<100mV), the recommended
frequency of 2.8V PWM signal should be above
2kHz. To fix the frequency of PWM signal and
change the duty cycle of PWM signal can get
different output current. According to the
application circuit of Figure 5, output current is
from 20.5mA to 5.5mA by adjusting the PWM duty
cycle from 10% to 90%.
VIN
22uH
1uF
1uF
ON
OFF
6
VIN
5
OUT
LP3303
4
CTRL
3
FB
PWM
(0-100KHz)
R3
100K
R1
5K
R2
90K
C1
0.1uF
Rset
10R
LP3303 –03 Ver. 1.3 Datasheet Feb.-2009
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