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Micrel, Inc
Functional Diagram
MIC3287
VIN
FB
OVP*
EN
OVP
V REF
V REF
250mV
g m
Error
Amplifier
PWM
Generator
SW
∑
Current
Amplifier
1.2MHz
Oscillator
Ramp
Generator
GND
* OV P available on TSOP-23-6 and MLF? package options onl y.
MIC3287 Block Diagram
I LED =
Functional Description
The MIC3287 is a constant frequency pulse width modulated
(PWM) current-mode step-up regulator. The block diagram
is shown above. The MIC3287 is composed of an oscillator,
slope compensation ramp generator, current amplifier, g m er-
ror amplifier, PWM generator, and a 350mA minimum bipolar
output transistor. The oscillator 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 com-
pensation 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 compensa -
tion ramp generator. This summed current-loop signal is fed
to one of the inputs of the PWM generator.
The g m error amplifier measures the LED current through the
external sense resistor and amplifies the error between the
detected signal and the internal 250mV 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, thus main-
taining the constant frequency current-mode PWM control.
The LED current is set by the feedback resistor:
250mV
R FB
The enable pin shuts down the internal power bipolar
transistor and disables the control circuitry to reduce input
current-to-leakage levels. Enable pin input current is zero
at zero volts.
April 2010
5
M9999-042910
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