MP3305 –36V, 1.3A, STEP-UP WHITE LED DRIVER WITH HIGH FREQUENCY TRUE PWM DIMMING & ADJUST OVP
MP3305 Rev. 1.0 www.MonolithicPower.com 6
4/10/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
OPERATION
The MP3305 uses a constant frequency, peak
current mode boost regulator architecture to
regulate the series string of white LEDs. The
operation of the MP3305 can be understood by
referring to the block diagram of Figure 1.
At the start of each oscillator cycle the FET is
turned on through the control circuitry. To
prevent sub-harmonic oscillations at duty cycles
greater than 50 percent, a stabilizing ramp is
added to the output of the current sense
amplifier and the result is fed into the positive
input of the PWM comparator. When this
voltage equals the output voltage of the error
amplifier the power FET is turned off.
The voltage at the output of the error amplifier
is an amplified version of the difference
between the 200mV reference voltage and the
feedback voltage. In this way the peak current
level keeps the output in regulation.
If the feedback voltage starts to drop, the output
of the error amplifier increases. This results in
more current flowing through the power FET,
thus increasing the power delivered to the
output.
PWM
COMPARATOR
AMPLIFIER
Enable
CONTROL
LOGIC
CURRENT
SENSE
AMPLIFIER
SW
M1
M2
PGND
OVP
0.9V
-
+
LED
ISET
ISET
EN
GND
2.2MHz
OSC
ENABLE &
DIMMING
CONTROL
-
+
-
+
+
-
+
Figure 1—Functional Block Diagram
