August 2004

7

M9999-081104

MIC2290

Micrel

Functional Description

The MIC2290 is a constant frequency, PWM current mode

boost regulator. The block diagram is shown in Figure 1. The

MIC2290 is composed of an oscillator, slope compensation

ramp generator, current amplifier, g

generator, and a 0.5A 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 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.

Functional Diagram

GND

CA

PWM

Generator

Ramp

Generator

1.2MHz

Oscillator

SW

EN

FB

OUT

VIN

1.24V

OVP

Σ

Figure 1. MIC2290 Block Diagram

The g

the external feedback resistors and amplifies the error be-

tween the detected signal and the 1.24V reference voltage.

The output of the g

signal that is fed to the other input of the PWM generator.

When the current-loop signal exceeds the voltage-loop sig-

nal, 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 con-

trol.