June 2005

5

M9999-062105

MIC4574

Micrel, Inc.

Functional Description

The MIC4574 is a variable duty cycle switch-mode regulator

with an internal power switch. Refer to the block diagrams.

Supply Voltage

The MIC4574 operates from a +4V to +24V unregulated

input. Highest efficiency operation is from a supply voltage

below +15V.

Enable/Shutdown

input if unused. A logic-low enables the regulator. A logic-high

shuts down the internal regulator which reduces the current

to typically 50µA.

Feedback

Fixed versions of the regulator have an internal resistive

output line.

Adjustable versions require an external resistive voltage

divider from the output voltage to ground, connected from the

Duty Cycle Control

A fixed-gain error amplifier compares the feedback signal

with a 1.23V bandgap voltage reference. The resulting error

amplifier output voltage is compared to a 200kHz sawtooth

waveform to produce a voltage controlled variable duty cycle

output.

A higher feedback voltage increases the error amplifier

output voltage. A higher error amplifier voltage (comparator

inverting input) causes the comparator to detect only the

peaks of the sawtooth, reducing the duty cycle of the com-

parator output. A lower feedback voltage increases the duty

cycle.

Output Switching

When the internal switch is on, an increasing current flows

from the supply V

output capacitor C

inductor as the current increases with time.

When the internal switch is turned off, the collapse of the

magnetic field in L1 forces current to flow through fast

recovery diode D1, charging C

Output Capacitor

External output capacitor C

reduces ripple.

Return Paths

During the on portion of the cycle, the output capacitor and

load currents return to the supply ground. During the off

portion of the cycle, current is being supplied to the output

capacitor and load by storage inductor L1, which means that

D1 is part of the high-current return path.