MP3205 – 1.3 MHz FIXED FREQUENCY 5 WHITE LED DRIVER

MP3205 Rev. 1.3

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3

7/13/2005

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TM

PIN FUNCTIONS

Pin #

Name Description

1

SW

Power Switch Output. SW is the drain of the internal MOSFET switch. Connect the power

inductor and output rectifier to SW. SW can swing between GND and 25V.

2

GND Ground.

3

FB

Feedback Input. The MP3205 regulates the voltage across the current sense resistor between

FB and GND. Connect a current sense resistor from the bottom of the LED string to GND.

Connect the bottom of the LED string to FB. The regulation voltage is 104mV.

4

EN

Regulator On/Off Control Input. A high input at EN turns on the converter, and a low input turns

it off. When not used, connect EN to the input source for automatic startup. The EN pin cannot

be left floating.

5

IN

Input Supply Pin. Must be locally bypassed.

OPERATION

The MP3205 uses a constant frequency, peak

current mode boost regulator architecture to

regulate the series string of white LEDs. The

operation of the MP3205 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 104mV 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 to flow

through the power FET, thus increasing the

power delivered to the output.

PWM

COMPARATOR

AMPLIFIER

CONTROL

LOGIC

CURRENT

SENSE

AMPLIFIER

SW

M1

GND

-

+

FB

1.3MHz

OSC

-

+

+

-

+

104mV

MP3205-F03-BD01

Figure 1—Functional Block Diagram