TM
MP3204 – 6V, 1.3MHz WHITE LED DRIVER
Open Load Protection
Open Load protection will shutoff the MP3204 if
the output voltage goes too high when the OV
pin is tied to the output. In some cases an LED
may fail, which will result in the feedback
voltage always being zero. The part will run at
the maximum duty cycle, boosting the output
voltage higher and higher. By tying the OV pin
to the top of the LED string the MP3204 checks
the said condition and if the output ever
exceeds 28V the MP3204 will shutdown. The
part will not switch on again until the power is
recycled. Figure 9 shows the behavior of the
MP3204 into an open load.
V
EN
5V/div.
Other applications require the use of a logic
signal to execute the dimming. This can be
seen in Figure 7. The PWM signal is applied to
the EN pin of the MP3204. The LEDs will switch
between full load to complete shutoff. The
average current through the LEDs will increase
proportionally to the duty cycle of the PWM
signal. The signal used in Figure 7 should be
1KHz or below due to the soft-start function.
LED1
LED2
LED3
PWM
4
MP3204
EN
FB
3
V
OV
10V/div.
MP3204_F07
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.
LED1
V
SW
10V/div.
MP3204_F09
Figure 9—Startup Waveforms into an Open
Load (V
IN
=3.6V)
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
EV3204DJ-00A for a sample layout of
theBMP3204.
MP3204
FB
3
LED2
LED3
PWM
MP3204_F08
Figure 8—Dimming Control Using a Filtered
PWM Signal
MP3204 Rev. 1.3
9/23/2005
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2005 MPS. All Rights Reserved.
7
MPS [ MONOLITHIC POWER SYSTEMS ]
