MP1518 – FIXED FREQUENCY WHITE LED DRIVER

MP1518 Rev. 1.6

www.MonolithicPower.com

6

7/25/2006

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TM

5V/div.

5V/div.

100mA/div.

Startup Waveforms

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 (%)

Figure 5—Current Limit vs. Duty Cycle

Setting the LED Current

The LED current is controlled by the feedback

resistor, R1, in Figure 6. The current through

the LEDs is given by the equation 104mV/R1.

Table 2 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 MP1518 during normal

operation. The first way uses a DC voltage to

control the feedback voltage. This 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

the resistor values shown for R2 and R3 can

control the LED current from 0mA to 20mA.

MP1518

LED1

LED2

LED3

FB

Figure 6—Dimming Control Using a DC

Voltage