MP3308 – STEP-UP WHITE LED DRIVER WITH DUAL PWM DIMMING INPUT CONTROL

MP3308 Rev. 1.02

www.MonolithicPower.com

9

7/19/2012

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© 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

PWMIB

DD

×

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

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.