EUP2571
Application Information
LED Current Control
The EUP2571 regulates the LED current by setting the
current sense resistor (R2) connecting to feedback and
ground. The internal feedback reference voltage is 0.25V.
The LED current can be set from following equation
easily.
0.25V
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.
Figure 4. PWM Dimming Control Using the EN Pin
R2 Resistor Value selection
b. Using a DC Voltage
ILED (mA)
R2 (Ω)
49.9
24.9
21
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 5.
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 5
sets dimming control of LED current from 20mA to
0mA.
5
10
12
15
20
16.5
12.4
Inductor Selection
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 EUP2571 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.
Dimming Control
a. Using a PWM Signal to EN Pin
For controlling the LED brightness, the EUP2571 can
perform the dimming control by applying a PWM
signal to EN pin. 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. Dimming Control Using a DC Voltage
Figure 6. Recommended Soft-Start Circuit
DS2571 Ver1.1 May. 2007
10
EUTECH [ EUTECH MICROELECTRONICS INC ]
