MIC2287 Micrel Semiconductor, MIC2287 Datasheet - Page 5

• Manufacturer Part Number: MIC2287
• Description: 1.2MHz PWM White LED Driver with OVP in 2mm 2mm MLF and Thin SOT-23
• Manufacturer: Micrel Semiconductor

Functional Diagram
Functional Description
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 amplifier, g
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 transis-
tor and to reset the slope compensation 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 compensation ramp generator. This summed
current-loop signal is fed to one of the inputs of the PWM
generator.
August 2004
MIC2287
*
OVP available on MLF
V
VIN
REF
95mV
FB
m
OVP*
error amplifier, PWM
OVP*
g
TM
m
package option only
Oscillator
1.2MHz
MIC2287 Block Diagram
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
M0510-081104
Micrel