EUP2571
Constant Output Voltage for Backlight of Main
Panel and Flashlight
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 7. 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 7, output current is from
20.5mA to 5.5mA by adjusting the PWM duty cycle
from 10% to 90%.
Figure 8 is an application of EUP2571 for backlight of
main panel and flashlight. Setting the divider-resistors
(R1 & R2) is to get a constant output voltage that
depends on the forward voltage and the numbers of
series-LEDs. There are three kinds of mode controlled
by the switches - backlight mode /flashlight mode
/backlight + flashlight mode. It can turn on backlight or
flashlight at one time or both at the same time.
Applying different duty cycle of PWM signal above
22kHz to backlight's switch can also control the
brightness. The following formula (2)(3) can determine
R3 and R4.
V
− 3VFb − V
OUT
DS
R3 =
R4 =
--------------------------(2)
--------------------------(3)
Ib
V
− 3VFf − V
OUT
DS
If
V
= Ib×R
---------------------------------(4)
DS(ON)
DS
Figure 7. Filtered PWM Signal for LED
Dimming Control
Figure 8. Constant Output Voltage for Backlight
and Flashlight
DS2571 Ver1.1 May. 2007
11
EUTECH [ EUTECH MICROELECTRONICS INC ]
