EUP2571
Constant Output Voltage for Backlight of Main
Panel and Keypad
Figure 9 is another application of EUP2571 for
backlight and keypad. 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.
It can turn on backlight of main panel and keypad at
the same time. Applying different duty cycle of PWM
signal above 22kHz to the backlight's switch can also
control the brightness of main panel's backlight. The
keypad's backlight will keep the same brightness
during the dimming control of main panel. Otherwise
the brightness of keypad's s backlight can also change
during the dimming control of main panel by using the
application circuit as figure 5. The following formula
(5)(6) can determine the resistors of Figure 9.
V
− 3VFb − V
OUT
DS
R3 =
----------------------------(5)
Ib
V
−3VFk
OUT
Ik
R4 = R5 = R6 =
----------------------(6)
V
= Ib×R
----------------------------------(7)
DS(ON)
DS
Figure 9. Constant Output Voltage for Backlight
and Keypad
DS2571 Ver1.1 May. 2007
12
EUTECH [ EUTECH MICROELECTRONICS INC ]
