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MP3302
MP3302 is a boost converter IC specifically
designed for LED drive applications. The
MP3302 can drive 27 LEDs (9 strings of 3
white LEDs in series) from a Lithium ion
battery. The IC has internal power MOSFETs
for driving the LEDs and has an efficiency of
88%. Switching frequency is 1.3MHz and the
internal current limit is 1.33A. Other features
of the MP3302 are open load shut down,
thermal shut down, under voltage lock out
etc. Applications of Mp3302 are LED back
lights, LED based lighting gadgets etc.
Functional block diagram of the MP3302 is
shown below (Fig1).
MP3302 functional block diagram
The IC uses a constant current , peak current
mode step up regulator scheme for regulating
the current through the LEDs. At
the beginning of each oscillator cycle, the
control circuitry switches the power MOSFET
ON. For preventing sub harmonic oscillation,
a stabilizing ramp signal is added to the
current sense amplifiers output and the
resultant signal is given to the non inverting
input of the PWM comparator. When this
resultant voltage is equal to the voltage at
the PWM comparator’s inverting input
(output voltage of the error amplifier), the
power MOSFET is switched OFF. The error
amplifier’s output is the difference between
the feedback voltage and the reference
voltage. When the output voltage drops, the
feedback voltage also drops and this in
increases the output of the error amplifier.
This in turn increases duty cycle of the power
MOSFET drive signal produced by the control
circuitry which increase the duty cycle of the
power MOSFET, it conducts more current and
the output voltage is regulated. Circuit
diagram of a 27 LED driver circuit using
MP3302 is shown below (Fig 2).
MP3302 LED driver circuit diagram.
MP3302 LED driver IC circuit
C1 is the input bypass capacitor and C2 is
the output bypass capacitor. Resistor R1 is
the feedback resistor and it controls the LED
current. The governing equation is: LED
current = 195mV /R1 . Resistors tagged R are
the current limiting resistors for the
corresponding strings and they can be used
for limiting the maximum brightness of the
LEDs. A voltage level less than 0.4V to the EN
pin will shut down the IC and a voltage level
greater than 0.7V will enable the IC. Dimming
of the LEDs can be achieved by providing a
PWM signal in the range of 200Hz to 1KHz to
the EN pin. The absolute minimum amplitude
of the PWM signal is 1.5V. The built in open
load protection circuit will shut down the IC
when ever the output voltage goes above
38V. The IC will remain in the shut down
mode until the power supply is re switched.
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