MP3304 –36V, 1.3A, STEP-UP WHITE LED DRIVER WITH HIGH FREQUENCY TRUE PWM DIMMING

MP3304 Rev. 0.91

www.MonolithicPower.com

6

4/19/2010

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OPERATION

The MP3304 uses a constant frequency, peak

current mode boost regulator architecture to

regulate the series string of white LEDs. The

operation of the MP3304 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

GND

-

+

LED

ISET

ISET

EN

2.2MHz

OSC

ENABLE &

DIMMING

CONTROL

-

+

+

-

+

200mV

M2

Figure 1—Functional Block Diagram