Micrel, Inc.

MIC2289

July 2007

5

M9999-071007

Functional Diagram

GND

PWM

Generator

Ramp

Generator

1.2MHz

Oscillator

SW

EN

FB

OUT

VIN

95mV

OVP

S

MIC2289 Block Diagram

Functional Description

The MIC2289 is a constant frequency, PWM current

mode boost regulator. The block diagram is shown

above. The MIC2289 is composed of an oscillator, slope

amplifier, PWM generator, 500mA bipolar output

transistor, and Schottky rectifier diode. 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 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.

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:

FB

LED

R

95mW

I

=

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.