MP3308 – STEP-UP WHITE LED DRIVER WITH DUAL PWM DIMMING INPUT CONTROL
MP3308 Rev. 1.02 www.MonolithicPower.com 9
7/19/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
OPERATION
The MP3308 uses a constant-frequency peak-
current–mode boost-regulator architecture to
regulate multiple strings of white LEDs connected
in series.
At the start of each oscillator cycle, the control
circuitry turns on the MOSFET. A stabilizing ramp
at the output of the current sense amplifier
prevents sub-harmonic oscillations at duty cycles
greater than 50 percent—this output feeds into
the positive input of the PWM comparator. When
this voltage equals the output voltage of the error
amplifier the power MOSFET turns off.
The error amplifier compares the difference
between the 200mV reference voltage and the
feedback voltage. If the feedback voltage drops,
the output of the error amplifier increases,
resulting in more current flowing through the
power MOSFET and increasing the power
delivered to the output, thus regulating the output.
Dual PWM dimming can adjust LED brightness.
True PWM dimming allows for flexible control of
the backlighting luminance to improve visibility for
a wide range of ambient brightness. A capacitor
on the FSET pin sets the PWM dimming
frequency with a duty cycle of:
PWMIA PWMIBDD×
Where D
PWMIA
is the duty cycle on the PWMIA pin,
and D
PWMIB
is the duty cycle on the PWMIB pin.
External capacitors on the PWMOA and the
PWMOB pins smooth the PWM signals to
provide DC signals by selecting the proper time
constant.
MP3308 provides over-voltage protection, which
will shut off the MP3308 if the output voltage
goes too high. OVP pin senses the output
voltage through the tap of a resistor divider
between V
OUT
and GND. In the case of an LED
failure, the feedback voltage goes to zero and the
part will run at maximum duty cycle. The output
voltage then rises and triggers OVP if the OVP
pin exceeds 1.23V.
MP3308 also provides short-circuit protection in
cases where individual LEDs on a string or a
whole LED string shorts: This condition causes
the ISET pin to pull high. If the voltage exceeds
0.4V for more than 240µs, MP3308 will shutdown.
