July 2009 6 M9999-070809
MIC2292/93 Micrel, Inc.
Functional Description
The MIC2292/93 is a constant frequency, PWM current mode
boost regulator. The block diagram is shown above. The
MIC2292/93 is composed of an oscillator, slope compensation
ramp generator, current amplier, g
m
error amplier, PWM
generator, 500mA bipolar output transistor, and Schottky
rectier diode. The oscillator generates a 1.6MHz clock for the
MIC2292 and a 2.0MHz clock for the MIC2293. The clocks'
two functions are to trigger the PWM generator that turns on
the output transistor and to reset the slope compensation
ramp generator. The current amplier is used to measure
the switch current by amplifying the voltage signal from the
internal sense resistor. The output of the current amplier
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
MIC2292/93 Block Diagram
The g
m
error amplier measures the LED current through the
external sense resistor and amplies the error between the
detected signal and the 95mV reference voltage. The output
of the g
m
error amplier 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
=
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.
GND
V
REF
PWM
Generator
Ramp
Generator
2.0MHz
Oscillator
SW
ENFB
OUT
VIN
95mV
g
m
OVP*
S
1.6MHz
or
