TM
MP1518 – FIXED FREQUENCY WHITE LED DRIVER
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
MP1518. 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.
Open Load Protection
Open Load protection will shut off the MP1518
if the output voltage goes 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 being 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 MP1518 checks this condition and if the
output ever exceeds 28V the MP1518 will shut
down. The part will not switch again until the
power is recycled. Figure 9 shows the behavior
of the MP1518 into an open load.
LED1
LED2
LED3
MP1518
EN
FB
PWM
Startup Waveforms into
an Open Load
V
= 3.6V
IN
V
EN
Figure 7—PWM Dimming Control Using a
Logic Signal
5V/div.
If the PWM signal is above 1KHz, dimming can
be achieved by using the circuit shown in
Figure 8.
V
OV
10V/div.
V
SW
10V/div.
LED1
LED2
LED3
MP1518
FB
Figure 9—Startup Waveforms into an Open
Load
Layout Considerations
PWM
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
EV0056 for a sample layout of the MP1518.
Figure 8— Dimming Control Using a Filtered
PWM Signal
MP1518 Rev. 1.6
7/25/2006
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