July 2005

5

M9999-070105

MIC2287

Micrel, Inc.

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

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

transistor and to reset the slope compensation ramp genera-

tor. 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.

Functional Diagram

GND

PWM

Generator

Ramp

Generator

1.2MHz

Oscillator

SW

EN

FB

OVP*

VIN

95mV

OVP*

S

MIC2287 Block Diagram

external sense resistor and amplifies the error between the

detected signal and the 95mV reference voltage. The output

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:

95mv

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.