lp3303 Lowpower Semiconductor inc, lp3303 Datasheet - Page 6

• Manufacturer Part Number: lp3303
• Description: High Performance, Constant Current Switching Regulator For 9pcs White Led
• Manufacturer: Lowpower Semiconductor inc

Applications Information
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.
I
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended). The
table for Rset selection is shown below.
ILED(mA)
20
15
12
10
5
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.
LP3303 –03 Ver. 1.3 Datasheet
LED(mA)
VIN
1uF
PWM
Rset Resistor Value selection
=200mV/Rset
22uH
6
4
VIN
CTRL
RSET(Ω)
10
13.3
16.6
20
40
LP3303
OUT
FB
5
3
Feb.-2009
1uF
Preliminary Datasheet
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.
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
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
application circuit of Figure 5, output current is
from 20.5mA to 5.5mA by adjusting the PWM duty
cycle from 10% to 90%.
ON
ON
VIN
VIN
1uF
OFF
1uF
OFF
22uH
ripple
22uH
output
(0-100KHz)
6
4
6
4
PWM
VIN
CTRL
VIN
CTRL
LP3303
LP3303
(<100mV),
current.
OUT
OUT
FB
FB
R3
100K
5
3
5
3
VDC
(0-2.8V)
According
the
1uF
R2
90K
1uF
R2
90K
C1
0.1uF
LP3303
R1
5K
R1
5K
Page 6 of 7
recommended
to
Rset
10R
Rset
10R
the