MP3202 – 1.3A FIXED FREQUENCY WHITE LED DRIVER

MP3202 Rev. 0.91

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7

9/21/2007

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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.

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 high frequency

switching, the length and area of all the traces

connected to the switch node should be

minimized.