EUP2538
Application Information
LED Current Control
The EUP2538 regulates the LED current by setting the
current sense resistor (R
FB
) connecting to feedback and
ground. The internal feedback reference voltage is 0.3V.
The LED current can be set from following equation
easily.
I LED
=
0.3
R FB
Switching Waveform with 1kHz PWM on EN
VEN
5V/div
VOUT
20V/div
LED
Current
20mA/div
250µs/div
--------------------------------------(1)
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended). The table
for R
FB
selection is shown below.
R
FB
Resistor Value selection
LED Current (mA)
5
10
15
20
25
30
Dimming Control
a. Using a PWM Signal to EN Pin
For controlling the LED brightness, the EUP2538 can
perform the dimming control by applying a PWM
signal to EN pin, and the PWM signal frequency range
is from 100Hz to 100KHz. 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.
R
FB
(Ω)
60
30
20
15
12
10
Figure 4.
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 5.
According to the Superposition Theorem, as the DC
voltage increases, the voltage contributed to V
FB
increases and the voltage drop on R2 decreases, i.e. the
LED current decreases. For example, if the V
DC
range
is from 0V to 2.8V, the selection of resistors in Figure 5
sets dimming control of LED current from 20mA to
0mA.
Figure 5. DC Voltage Dimming Control
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 6, output current is from
20.5mA to 5.5mA by adjusting the PWM duty cycle
from 10% to 90%.
Figure 3. Direct PWM Dimming Control
DS2538
Ver1.3
Nov. 2008
8
EUTECH [ EUTECH MICROELECTRONICS INC ]
