August, 2004

5

M9999-081104

MIC2291

Micrel

Functional Description

The MIC2291 is a constant frequency, PWM current mode

boost regulator. The block diagram is shown above. The

MIC2291 is composed of an oscillator, slope compensation

ramp generator, current amplifier, g

generator,

and a 500mA bipolar output transistor. The

oscillator generates a 1.2MHz clock. The clock’s two func-

tions 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

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

PWM

Generator

Ramp

Generator

1.2MHz

Oscillator

SW

EN

FB

OVP*

VIN

95mV

OVP*

Σ

Figure 1. MIC2291 Block Diagram

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:

I

95mv

R

FB

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.