MP3205 – 1.3 MHz FIXED FREQUENCY 5 WHITE LED DRIVER

MP3205 Rev. 1.3

www.MonolithicPower.com

4

7/13/2005

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2005 MPS. All Rights Reserved.

TM

APPLICATION INFORMATION

OFF ON

MP3205

2.5V to 6V

LED1

LED2

LED3

MP3205_F02_AP_S01

CMDSH-3

IN

EN

GND

4

2

FB

3

SW

51

Figure 2—Driving 3 White LEDs

COMPONENT SELECTION

A typical application circuit can be seen in Figure

2. The 3 white LEDs can be driven from a voltage

supply range of 2.5V to 6V at an output current of

20mA. A 0.22µF output capacitor is sufficient for

most applications but an output capacitor up to

1µF may be used. A 22µH inductor with low DCR

(Inductor resistance) is recommended to improve

efficiency.

A

1µF

ceramic

capacitor

is

recommended for the input capacitance in the

real system. Schottky diodes have fast recovery

and a low forward voltage and are recommended.

Schottky diodes rated with 100mA to 200mA

are sufficient for the MP3205. The switching

characteristics during normal operation can be

seen in Figure 3. The MP3205 has internal

soft-start to limit the amount of current through

overshoot on the output. The current limit is

increased by a fourth every 40µs giving a total

soft-start time of 120µs.

(AC COUPLED)

50mV/div.

5V/div.

100mA/div.

MP3205_F03_WF01

Figure 3—Steady-State Operation

Figure 4 shows the startup behavior of the

MP3205. 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.

MP3205_F04_WF02

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 (%)

MP3205_F05

Figure 5—Current Limit vs. Duty Cycle