Micrel, Inc.

MIC2290

October 2007

7

M9999-101907

Functional Diagram

GND

CA

VREF

PWM

Generator

Ramp

Generator

1.2MHz

Oscillator

SW

EN

FB

OUT

VIN

1.24V

gm

OVP

S

Figure 1. MIC2290 Block Diagram

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,

gm error amplifier, PWM 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.

The gm error amplifier measures the feedback voltage

through the external feedback resistors and amplifies the

error between the detected signal and the 1.24V

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.