EUP2518
Application Information
LED Current Control
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 RFB decreases, i.e.
the LED current decreases. For example, if the VDC
range is from 0V to 2V, the selection of resistors in
Figure 4 sets dimming control of LED current from
20mA to 0mA.
The EUP2518 regulates the LED current by setting the
current sense resistor (RFB) connecting to feedback and
ground. The internal feedback reference voltage is
104mV. The LED current can be set from following
equation easily.
104mV
R
=
-------------------------------------(1)
I
FB
LED
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended). The table
for RFB selection is shown below.
RFB Resistor Value selection
ILED (mA)
RFB (Ω)
5
21
10
15
20
10.4
6.93
5.2
CAP and Inductor Selection
The recommended value of inductor for 2 to 6 WLEDs
applications are 10 H to 47 H. Small size and better
efficiency are the major concerns for portable device. A
22 H inductor with low DCR (Inductor resistance) is
recommended to improve efficiency. To avoid inductor
saturation current rating should be considered. A 1
ceramic capacitor is recommended for the input
capacitance in the real system, and a larger capacitor
will get smaller input voltage ripple. A 0.22uF output
ceramic capacitor is sufficient for most applications.
µ
µ
Figure 4. Dimming Control Using a DC Voltage
µ
c. Using a Filtered PWM signal
µF
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. In this circuit, the output
ripple depends on the frequency of PWM signal. For
smaller output voltage ripple (<100mV), the
recommended frequency of 2V 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 EUP2518 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
DS2518 Ver1.0 Mar. 2008
6
EUTECH [ EUTECH MICROELECTRONICS INC ]
