MP1518 – FIXED FREQUENCY WHITE LED DRIVER

MP1518 Rev. 1.6

www.MonolithicPower.com

7

7/25/2006

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TM

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.

PWM

MP1518

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 by using the circuit shown in

Figure 8.

PWM

MP1518

LED1

LED2

LED3

FB

Figure 8— Dimming Control Using a Filtered

PWM Signal

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.

5V/div.

10V/div.

10V/div.

Startup Waveforms into

an Open Load

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

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.