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MIC2287 Datasheet(Arkusz danych) 5 Page - Micrel Semiconductor

Numer części MIC2287
Szczegółowy opis  1.2MHz PWM White LED Driver with OVP in 2mm × 2mm MLF™ and Thin SOT-23
Pobierz  10 Pages
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Producent  MICREL [Micrel Semiconductor]
Strona internetowa  http://www.micrel.com
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 5 page
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August 2004
5
M0510-081104
MIC2287
Micrel
Functional Description
The MIC2287 is a constant frequency, PWM current mode
boost regulator. The block diagram is shown above. The
MIC2287 is composed of an oscillator, slope compensation
ramp generator, current amplifier, g
m error amplifier, PWM
generator, and a 500mA bipolar output transistor. The oscil-
lator generates a 1.2MHz clock. The clock’s two functions are
to trigger the PWM generator that turns on the output transis-
tor and to reset the slope compensation ramp generator. The
current amplifier is used to measure the switch current by
amplifying the voltage signal from the internal sense resistor.
The output of the current amplifier is summed with the output
of the slope compensation ramp generator. This summed
current-loop signal is fed to one of the inputs of the PWM
generator.
Functional Diagram
GND
VREF
PWM
Generator
Ramp
Generator
1.2MHz
Oscillator
SW
EN
FB
OVP*
VIN
95mV
*OVP available on MLFTM package option only
gm
OVP*
Σ
MIC2287 Block Diagram
The g
m error amplifier measures the LED current through the
external sense resistor and amplifies the error between the
detected signal and the 95mV reference voltage. The output
of the g
m error amplifier provides the voltage-loop signal that
is fed to the other input of the PWM generator. When the
current-loop signal exceeds the voltage-loop signal, the
PWM generator turns off the bipolar output transistor. The
next clock period initiates the next switching cycle, maintain-
ing the constant frequency current-mode PWM control. The
LED is set by the feedback resistor:
I
LED =
95mv
R
FB
The Enable pin shuts down the output switching and disables
control circuitry to reduce input current-to-leakage levels.
Enable pin input current is zero at zero volts.




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