LP3385 Datasheet, PDF (7/9 Pages) Lowpower Semiconductor inc – 1.2MHzFixed-Frequency PWM Operation

Preliminary Datasheet
PWM signal should be higher than the maximum
enable voltage of EN pin, in order to let the
dimming control perform correctly.
Vin
Cin
10uF
22uH
U77
6 _{Vin OVP} 5
LP3385
Cout
1uF
Vin
Cin
10uF
22uH
U80
6 _{Vin OVP} 5
PWM
4 _EN
FB ³
LP3385
Cout
1uF
VFB=300mV
4 _EN
FB ³
LP3385
R3
R2
VDC
90k
5k
R1
15
c. Using a Filtered PWM signal
Another common application is using a filtered
R1
15
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%.
Vin
Cin
10uF
22uH
U78
Cout
1uF
b. Using a DC Voltage
Using a variable DC voltage to adjust the brightness
6 _{Vin OVP} 5
is a popular method in some applications. The
dimming control using a DC voltage circuit is
shown in Figure 4. According to the Superposition
4 _EN
FB ³
LP3385
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
R2
R3 ^5k
R1
R4
15
PWM
90k
100K
Cout
0.1uF
selection of resistors in Figure 4 sets dimming
control of LED current from 20mA to 0mA.
LP3385 – 00 Version 1.0 Datasheet FEB.-2012
www.lowpowersemi.com
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