MP3205 – 1.3 MHz FIXED FREQUENCY 5 WHITE LED DRIVER
MP3205 Rev. 1.3 www.MonolithicPower.com 5
10/8/2007 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
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TM
Setting the LED Current
The LED current is controlled by the feedback
resistor, R1, in Figure 2. The current through
the LEDs is given by the equation 104mV/R1.
Table 1 shows the selection of resistors for a
given LED current.
Table 1—I
LED
vs. R1
I
LED
(mA) R1 (Ω)
1 104
5 20.8
10 10.4
15 6.93
20 5.2
Analog and Digital Dimming
There are three different ways to control dimming
for the MP3205 during normal operation. The first
way uses a DC voltage to control the feedback
voltage. This can be seen in Figure 6. As the DC
voltage increases, current starts flowing down R1,
R2 and R3. The loop will continue to regulate the
feedback voltage to 104mV. Thus the current has
to decrease through the LEDs by the same
amount of current as is being injected from the
DC voltage source. With a V
DC
from 0V to 2V, the
resistor values shown for R2 and R3 can control
the LED current from 0mA to 20mA.
MP3205
LED1
LED2
LED3
FB
3
V
DC
MP3205_F06_S01
Figure 6—Dimming Control Using a DC Voltage
Other applications need to use a logic signal to
do the dimming. This can be seen in Figure 7.
The PWM signal is applied to the EN pin of the
MP3205. The LEDs will switch between full load
to completely shut off. The average current
through the LEDs will increase proportionally to
the duty cycle of the PWM signal. The PWM
signal used in Figure 7 should be 1KHz or
below due to the soft start function.
PWM
MP3205
LED1
LED2
LED3
FB
3
MP3205_F07_S01
EN
4
Figure 7—PWM Dimming Control Using a
Logic Signal
If the PWM signal is above 1KHz, dimming can
be achieved by using the circuit shown in
Figure 8.
PWM
MP3205
LED1
LED2
LED3
FB
3
MP3205_F08_S01
Figure 8—Dimming Control Using a Filtered
PWM Signal
Layout Considerations
Careful attention must be paid to the PCB
board layout and component placement. Proper
layout of the high frequency switching path is
critical to prevent noise and electromagnetic
interference problems. Due to the high
frequency switching the length and area of all
the traces connected to the switch node should
be minimized. Refer to the evaluation board
EV3205DJ-00A for a sample layout of
theBMP3205.
