MP3389—12-STRING WHITE LED DRIVER WITH STEP-UP CONTROLLER

MP3389 Rev. 1.04

www.MonolithicPower.com

13

3/27/2013

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BOSC

DBRT

MP3389

PWM Dimming

100Hz~2kHz

R1

100 k

Figure 4—Direct PWM Dimming

Table 1 shows the PWM dimming duty Range

with different PWM dimming frequency.

Tab 1 The Range of PWM Dimming Duty

fpwm(Hz)

Dmin

Dmax

100<f≤200

0.30%

100%

200<f≤500

0.75%

100%

500<f≤1k

1.50%

100%

1k<f≤2k

3.00%

100%

2k<f≤5k

7.50%

100%

5k<f≤10k

15.00%

100%

10k<f≤20k

30.00%

100%

2. DC Input PWM Dimming

To apply DC input PWM dimming, apply an

analog signal (range from 0.2 V to 1.2V) to the

DBRT pin to modulate the LED current directly. If

the PWM is applied with a zero DC voltage, the

PWM duty cycle will be 0%. If the DBRT pin is

applied with a DC voltage>1.2V, the output will

be 100% (See Figure 5). The capacitor on BOSC

pin set the frequency of internal triangle

waveform according to the equation:

C1

BOSC

DBRT

MP3389

DC Signal

Figure 5—DC input PWM Dimming

Layout Considerations

Careful attention must be paid to the PCB board

layout and components placement. Proper layout

of the high frequency switching path is critical to

prevent noise and electromagnetic interference

problems. The loop of external MOSFET (M2),

output diode (D1), and output capacitor (C5) is

flowing with high frequency pulse current. it must

be as short as possible (See Figure 6).

Figure 6—Layout Consideration

The IC exposed pad is internally connected to

GND pin, and all logic signals are refer to the

GND. The PGND should be externally connected

to GND and is recommended to keep away from

the logic signals.

External MOSFET for Short Protection

When output voltage is higher than rating of

LED1-12 pins, external MOSFET is needed for

each LED channel to avoid destroying LED1 -12

pins. (See Figure 8 for 4 channels application)

The rating of MOSFET must be higher than 1.2*