MP3202 – 1.3A FIXED FREQUENCY WHITE LED DRIVER
MP3202 Rev. 0.91 www.MonolithicPower.com 7
9/21/2007 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2007 MPS. All Rights Reserved.
Other applications require a logic signal to
control dimming. This is shown in Figure 7. The
PWM signal is applied to the EN pin of the
MP3202. 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
MP3202
LED1
LED2
LED3
FB
EN
Figure 7—PWM Dimming Control Using a
Logic Signal
If the PWM signal is above 1KHz, dimming can
be achieved using the circuit shown in Figure 8.
PWM
MP3202
LED1
LED2
LED3
FB
Figure 8— Dimming Control Using a Filtered
PWM Signal
Open Load Protection
Open Load protection will shut off the MP3202
if the output voltage rises too high when the OV
pin is tied to the output. In some cases an LED
may fail. This will result in the feedback voltage
always remaining at zero.
The part will run at maximum duty cycle
boosting the output voltage higher and higher.
By tying the OV pin to the top of the LED string
the MP3202 can check for this condition. If the
output exceeds 28V, the MP3202 will shut
down. The part will not switch again until the
power is recycled. Figure 9 shows the behavior
of the MP3202 into an open load.
V
EN
5V/div.
V
OV
10V/div.
V
SW
10V/div.
Startup Waveforms into
an Open Load
V
IN
= 3.6V
Figure 9—Startup Waveforms into an Open
Load
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 high frequency
switching, the length and area of all the traces
connected to the switch node should be
minimized.
