MP3305 –36V, 1.3A, STEP-UP WHITE LED DRIVER WITH HIGH FREQUENCY TRUE PWM DIMMING & ADJUST OVP

MP3305 Rev. 1.0

www.MonolithicPower.com

7

4/10/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

Enable/

Dimming

Control

MP3305

5V

MBR0520

C2

C1

IN

EN

GND

LED

ISET

OVP

SW

Figure 2—Circuit for Driving 10 Strings of 3 WLEDs

A typical application circuit can be seen in Figure

2. The 10 strings of 3 white LEDs can be driven

from a 5V at an output current of 200mA. A 2.2µF

output capacitor is sufficient for most applications.

A 2.2µH inductor with low DCR (Inductor DC

resistance)

is

recommended

to

improve

efficiency.

A

10µF

ceramic

capacitor

is

recommended for the input capacitance in the

real system. Schottky diodes have fast recovery

and a low forward voltage are recommended.

Schottky diodes rated with 500mA are sufficient

for the MP3305. The MP3305 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 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.

Setting the LED Current

The LED current is controlled by the current

setting resistor, R1. The current through the

Table 1 shows the selection of resistors for a

given LED current.

R1 (Ω)

1

200

5

40

10

20

20

10

60

3.33

180

1.11

200

1

PWM Dimming

Apply a 250Hz to 50kHz square waveform to the

EN pin to implement PWM dimming of the LEDs.

The minimum recommended amplitude of the

PWM signal is 2V.

Open Load Protection

Open Load protection will shut off the MP3305 if

the output voltage goes too high. In some cases

an LED may fail, this will result in the feedback

voltage always being zero. The part will run at

maximum duty cycle boosting the output voltage

higher and higher. If the output voltage ever

exceeds

the

adjusted

OVP

threshold

(

+

OV

R2

V =0.9V(1

)

R3

), the MP3305 will shut down.

The part will not switch again until the power is

recycled.