EUP2584A
Application Information
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.
LED Current Control
The EUP2584A regulates the LED current by setting the
current sense resistor (R2) connecting to feedback and
ground. The internal feedback reference voltage is 0.3V.
The LED current can be set from following equation
easily.
0.3V
R2 =
--------------------------------------(1)
I
LED
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended). The table
for R2 selection is shown below.
R2 Resistor Value selection
R2 (Ω)
60
I
LED (mA)
5
10
12
15
20
30
25
20
15
Inductor Selection
Figure 4. Dimming Control Using a DC Voltage
The recommended value of inductor for 2 to 6 WLEDs
applications are 4.7 to 22µH. Small size and better
efficiency are the major concerns for portable device,
such as EUP2584A used for mobile phone. The inductor
should have low core loss at 1.1MHz and low DCR for
better efficiency. To avoid inductor saturation current
rating should be considered.
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 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%.
Dimming Control
a. Using a PWM Signal to EN Pin
For controlling the LED brightness, the EUP2584A can
perform the dimming control by applying a PWM
signal to EN pin. The internal soft-start and wide range
dimming frequency from 100Hz to 100KHz can
insignificantly reduce audio noise when dimming. The
average LED current is proportional to the PWM signal
duty cycle. The 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.
Figure 5. Filtered PWM Signal for LED Dimming
Control
Figure 3. PWM Dimming Control Using the EN Pin
DS2584A Ver1.0 Nov. 2007
9
EUTECH [ EUTECH MICROELECTRONICS INC ]
