MIC2291_07 Datasheet, PDF (6/10 Pages) Micrel Semiconductor – 1.2A PWM Boost Regulator Photo Flash LED Driver

Micrel, Inc.
Functional Diagram
VIN FB OVP*
EN
OVP*
g_m
V_REF
95mV
PWM
Generator
1.2MHz
Oscillator
Ramp
Generator
*OVP available on MLF^® package option only
Figure 1. MIC2291 Block Diagram
SW
GND
MIC2291
Functional Description
The MIC2291 is a constant frequency, PWM current
mode boost regulator. The block diagram is shown
above. The MIC2291 is composed of an oscillator, slope
compensation ramp generator, current amplifier, g_m error
amplifier, PWM generator, and a 500mA 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
compensation ramp generator. The current amplifier is
used to measure the switch cur-rent 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.
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 gm 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, maintaining the constant
frequency current-mode PWM control. The LED is set by
the feedback resistor:
I_LED
=
95mW
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.
May 2007
6
M9999-051507