Micrel, Inc.

MIC2289C

August 2007

5

M9999-081507-B

Functional Diagram

GND

PWM

Generator

Ramp

Generator

1.2MHz

Oscillator

SW

EN

FB

OUT

VIN

95mV

OVP

S

MIC2289C Block Diagram

Functional Description

The MIC2289C is a constant frequency, PWM current

mode boost regulator. The block diagram is shown

above. The MIC2289C is composed of an oscillator,

slope compensation ramp generator, current amplifier,

gm error 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 gm error amplifier measures the LED current

through 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.