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MIC2287
Functional Diagram
Micrel, Inc.
VIN
FB
OVP*
EN
OVP*
SW
V REF
95mV
g m
PWM
Generator
S
1.2MHz
Oscillator
Ramp
Generator
GND
* OVP available on MLF TM package option only
MIC2287 Block Diagram
I LED =
FunctionalDescription
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 ampli?er, g m error ampli?er, 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
transistor and to reset the slope compensation ramp genera-
tor. The current ampli?er is used to measure the switch cur-
rent by amplifying the voltage signal from the internal sense
resistor. The output of the current ampli?er 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.
The g m error ampli?er measures the LED current through the
external sense resistor and ampli?es the error between the
detected signal and the 95mV reference voltage. The output
of the g m error ampli?er 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:
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.
July 2005
5
M9999-070105
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