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LP3321 数据表(PDF) 6 Page - Lowpower Semiconductor inc

部件名 LP3321
功能描述  1.3MHzFixed-Frequency PWM Operation
PDF  9 Pages
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制造商  POWER [Lowpower Semiconductor inc]
网页  http://www.lowpowersemi.com
标志 POWER - Lowpower Semiconductor inc

LP3321 数据表(HTML) 6 Page - Lowpower Semiconductor inc

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Preliminary Datasheet
LP3321
LP3321 – 01 Vesion 1.1 Datasheet
Feb.-2011
www.lowpowersemi.com
Page 6 of 9
Applications Information
LED Current Control
The LP3321 regulates the LED current by setting
the current sense resistor (R1) connecting to
feedback
and
ground.
The
internal
feedback
reference voltage is 195mV or LP3321A is 300mV.
The LED current can be set from following equation
easily.
ILED=195mV/R1 (LP3321)
ILED=300mV/R1 (LP3321A)
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended). The
table for R1 selection is shown below.
Inductor Selection
The recommended value of inductor for 2 to 10
WLEDs applications are 4.7 to 22µH. Small size
and better efficiency are the major concerns for
portable device, such as LP3321 used for mobile
phone. The inductor should have low core loss at
1.3MHz and low DCR for better efficiency. To
avoid inductor saturation current rating should be
considered.
Dimming control
a. Using a PWM Signal to EN Pin
For controlling the LED brightness, the LP3321 can
perform the dimming control by applying a PWM
signal to EN pin. The internal soft-start and wide
range dimming frequency from 100Hz to 100KHz
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 EN pin, in order to let the
dimming control perform correctly.
PWM
Cout
1uF
10uH
D1
SS14
Cin
10uF
Vin
VFB=195mV
LP3321
4
3
5
EN
FB
Vin
R1
10
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 R2 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.
VDC
LP3321
4
3
5
EN
FB
Vin
R2
5k
Vin
R1
10
Cout
1uF
Cin
10uF
10uH
R3
90k
D1
SS14



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