MIC2291-15BML MICREL [Micrel Semiconductor], MIC2291-15BML Datasheet - Page 5

• Manufacturer Part Number: MIC2291-15BML
• Description: 1.2A PWM Boost Regulator Photo Flash LED Driver
• Manufacturer: MICREL [Micrel Semiconductor]

Functional Diagram
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
generator,
oscillator generates a 1.2MHz clock. The clock’s two func-
tions are to trigger the PWM generator that turns on the output
transistor and to reset the slope compensation ramp genera-
tor. 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.
August, 2004
MIC2291
and a 500mA bipolar output transistor. The
*
OVP available on MLF
V
VIN
REF
95mV
FB
m
OVP*
error amplifier, PWM
OVP*
g
TM
m
Figure 1. MIC2291 Block Diagram
package option only
Oscillator
1.2MHz
5
The g
external sense resistor and amplifies the error between the
detected signal and the 95mV reference voltage. The output
of the g
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:
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.
Generator
Generator
Ramp
PWM
EN
m
I
LED
m
error amplifier measures the LED current through the
error amplifier provides the voltage-loop signal that
=
95mv
R
FB
SW
GND
M9999-081104
Micrel