April 1998•

4-133

MIC4576

MIC4576

Micrel

Functional Description

The MIC4576 is a variable duty cycle switch-mode regulator

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

Supply Voltage

The MIC4576 operates from a +4V to +36V unregulated

input. Highest efficiency operation is from a supply voltage

around +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 50

µA.

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.

Applications Information

The applications circuits that follow have been constructed

and tested.

Refer to Application Note 15 for additional

information, including efficiency graphs and manufacturer’s

addresses and telephone numbers for most circuits.

For a mathematical approach to component selection and

circuit design, refer to Application Note 14.

C1

470µF

35V

D1

1N5822

C2

1000µF

16V

L1

33µH

GND

FB

SW

MIC4576-3.3BT

SHDN

6V to 24V

3.3V/3A

C1 Nichicon

UPL1V471MPH, ESR = 0.046

Ω

C2 Nichicon

UPL1C221MPH, ESR = 0.047

Ω

D1 Motorola

1N5822

L1

Coiltronics PL52C-33-1000, DCR = 0.036

Ω

L1

Bi

HM77-30004, DCR = 0.045

Ω, Note 2

3

4

2

5

1

Figure 1. 6V–24V to 3.3V/3A Buck Converter

Through Hole

C1

470µF

63V

D1

MBR360

C2

1000µF

16V

L1

33µH

GND

FB

SW

MIC4576-3.3BT

SHDN

6V to 36V

3.3V/3A

C1 Nichicon

UPL1J471MRH, ESR = 0.039

Ω

C2 Nichicon

UPL1C102MPH, ESR = 0.047

Ω

D1 Motorola

MBR360

L1

Coiltronics PL52C-33-1000 DCR = 0.036

L1

Bi

HM77-30004, DCR = 0.045

Ω, Note 2

3

4

2

5

1

Figure 2. 6V–36V to 3.3V/3A Buck Converter

Through Hole

Note 2:

Surface-mount component.