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 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
APPLICATION INFORMATION
Enable/
Dimming
Control
MP3305
V
IN
5V
MBR0520
C2
C1
IN
EN
GND
LED
ISETOVP
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
LEDs is given by: I
LED
=200mV/R1.
Table 1 shows the selection of resistors for a
given LED current.
Table 1—I
LED
vs. R1
I
LED
(mA) 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.
