MP1518 – FIXED FREQUENCY WHITE LED DRIVER

MP1518 Rev. 1.6

www.MonolithicPower.com

5

7/25/2006

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TM

APPLICATION INFORMATION

OFF ON

MP1518

2.5V to 6V

LED1

LED2

LED3

CMDSH-3

IN

EN

GND

OV

FB

SW

Figure 2—Circuit for Driving 3 WLEDs

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 MP1518.

The switching characteristics during normal

operation can be seen in Figure 3. The MP1518

has internal soft-start to limit the amount of

current through VIN at startup and to also limit

the amount of overshoot on the output. The

current limit is increased by a fourth every 40µs

giving a total soft-start time of 120µs.

5V/div.

50mV/div.

100mA/div.

200ns/div.

Steady State Operation

Figure 3—Steady State Operation

Figure 4 shows the startup behavior of the

MP1518. 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.