TM
MP3205 – 1.3 MHz FIXED FREQUENCY 5 WHITE LED DRIVER
PIN FUNCTIONS
Pin #
1
2
3
Name Description
SW
GND
FB
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.
Ground.
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.
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.
Input Supply Pin. Must be locally bypassed.
4
5
EN
IN
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.
FB
+
-
-
+
AMPLIFIER
CONTROL
LOGIC
M1
SW
104mV
PWM
COMPARATOR
-
+
+
CURRENT
SENSE
AMPLIFIER
1.3MHz
OSC
MP3205-F03-BD01
GND
Figure 1—Functional Block Diagram
3
MP3205 Rev. 1.3
7/13/2005
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