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LP3385 数据表(PDF) 7 Page - Lowpower Semiconductor inc

部件名 LP3385
功能描述  1.2MHzFixed-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

LP3385 数据表(HTML) 7 Page - Lowpower Semiconductor inc

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Preliminary Datasheet
LP3385
LP3385 – 00 Version 1.0 Datasheet
FEB.-2012
www.lowpowersemi.com
Page 7 of 9
PWM signal should be higher than the maximum
enable voltage of EN pin, in order to let the
dimming control perform correctly.
Cout
1uF
22uH
Cin
10uF
R1
15
Vin
VFB=300mV
PWM
EN
4
FB
3
Vin
6
OVP
5
U80
LP3385
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.
Cout
1uF
22uH
Cin
10uF
R1
15
Vin
R2
5k
R3
90k
VDC
EN
4
FB
3
Vin
6
OVP
5
U77
LP3385
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
recomended 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%.
PWM
Vin
R2
5k
R1
15
Cout
1uF
Cin
10uF
22uH
R3
90k
Cout
0.1uF
R4
100K
EN
4
FB
3
Vin
6
OVP
5
U78
LP3385



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