LP3382S Datasheet, PDF (7/9 Pages) Lowpower Semiconductor inc – 1.2MHzFixed-Frequency PWM Operation

Preliminary Datasheet
LP3382S
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.
selection of resistors in Figure 4 sets dimming
control of LED current from 20mA to 0mA.
Vin
Cin
10uF
4.7uH
SS0520
Cout
1uF
Vin
Cin
10uF
4.7uH
SS14
PWM
0-1MHz
6 _{Vin OVP} 5
4
EN
R2
100k
3
FB
LP3382S
Cout
1uF
6 _{Vin OVP} 5
ON/OFF
4 _EN
R4
100k
FB ³
LP3382S
R3
R2
VDC
56K
6.8K
R1
12
c. Using a Filtered PWM signal
R1 _{Another common application is using a filtered}
12
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
4.7uH
SS0520
Cout
1uF
b. Using a DC Voltage
6 _Vin
OVP ⁵
Using a variable DC voltage to adjust the brightness
is a popular method in some applications. The
ON/OFF
4 _EN
FB ³
dimming control using a DC voltage circuit is
R4
100k
LP3382S
shown in Figure 4. According to the Superposition
R2
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
R3
6.8K
R1
PWM
R69
56K
15
(0-100KHz) ^100K
C3
0.1uF
LP3382S – 00 Version 1.0 Datasheet OCT.-2012
www.lowpowersemi.com
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