MP3202 – 1.3A FIXED FREQUENCY WHITE LED DRIVER

MP3202 Rev. 0.91

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6

9/21/2007

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5V/div.

5V/div.

200mA/div.

500mA/div.

Startup Waveforms

Figure 4—Startup Waveforms

Figure 5 shows the dependence on current limit

versus duty cycle.

1.6

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0

20406080

100

DUTY CYCLE (%)

Figure 5—Current Limit vs. Duty Cycle

Setting the LED Current

The LED current is controlled by the feedback

resistors R1 and R2, as shown in Figure 2. The

current through the LEDs is given by the

equation:

2

R

1

R

2

R

1

R

mV

104

×

+

×

=

Table 1 shows the selection of resistors for a

given LED current.

R1 (Ω)

R2 (Ω)

10

10.4

N/A

50

2.08

N/A

100

1.04

N/A

150

1.5

1.3

180

1.2

1.1

Analog and Digital Dimming

There are three methods to control dimming for

the MP3202 during normal operation. The first

method uses DC voltage to control the

feedback voltage. This is shown in Figure 6. As

the DC voltage increases, current starts flowing

down R1, R2 and R3. The loop will continue to

regulate the feedback voltage to 104mV. Thus

the current has to decrease through the LEDs

by the same amount of current as is being

from 0V to 2V, the resistor values shown for R2

and R3 can control the LED current from 0mA

to 20mA.

MP3202

LED1

LED2

LED3

FB

Figure 6—Dimming Control Using a DC

Voltage