MP3204 – 6V, 1.3MHz WHITE LED DRIVER

MP3204 Rev. 1.3

www.MonolithicPower.com

6

9/23/2005

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© 2005 MPS. All Rights Reserved.

TM

Figure 4 shows the startup behavior of the

MP3204. The ramped voltage that is added to

the current sense amplifier reduces the current

output as the duty cycle increases. As more

LEDs are added, the output voltage rises but

the current that can be delivered to the load is

reduced as well.

5V/div.

5V/div.

100mA/div.

MP3204_F04

Figure 4—Startup Waveforms

Figure 5 shows the dependence on current limit

versus duty cycle.

500

400

300

200

100

0

0

20406080

100

DUTY CYCLE (%)

MP3204_F05

Figure 5—Current Limit vs. Duty Cycle

Setting the LED Current

The LED current is controlled by the feedback

resistor, R1, as shown in Figure 2. The current

through the LEDs is equal to 104mV/R1. Table

1 shows the selection of resistors for a given

LED current.

R1 (Ω)

1

104

5

20.8

10

10.4

15

6.93

20

5.2

Analog and Digital Dimming

There are three different ways to control

dimming for the MP3204 during normal

operation. The first way uses a DC voltage to

control the feedback voltage, which can be

seen 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 injected from the

DC voltage source. With a VDC from 0V to 2V,

the resistor values shown for R2 and R3 can

control the LED current from 0mA to 20mA.

MP3204

LED1

LED2

LED3

FB

3

MP3204_F06

Figure 6—Dimming Control Using a DC

Voltage