MIC4574

Micrel

4-96

April 1998•

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 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

47µF

35V

D1

1N5819

C2

220µF

16V

L1

100µH

SGND

FB

SW

MIC4574-3.3BN

SHDN

6V to 24V

3.3V/0.5A

C1 Nichicon

UPL1V470MEH, ESR = 0.34

Ω

C2 Nichicon

UPL1C221MPH, ESR = 0.16

Ω

D1 Motorola

1N5819

L1

Sumida

RCH875-101K, DCR = 0.28

Ω

PGND

24

1

7

3

5

Figure 1. 6V–24V to 3.3V/0.5A Buck Converter

Through Hole

C1

47µF

35V

D1

1N5819

C2

220µF

16V

L1

100µH

SGND

FB

SW

MIC4574-5.0BN

SHDN

8V to 24V

5.0V/0.5A

C1 Nichicon

UPL1J470MEH, ESR = 0.34

Ω

C2 Nichicon

UPL1C221MPH, ESR = 0.16

Ω

D1 Motorola

1N5819

L1

Sumida

RCH875-101K, DCR = 0.28

Ω

PGND

24

1

7

3

5

Figure 2. 8V–24V to 5V/0.5A Buck Converter

Through Hole

C1

33µF

63V

MBR160

C2

220µF

16V

L1

220µH

SGND

FB

SW

MIC4574BN

SHDN

16V to 24V

12V/0.5A

R2

13.0k

1%

R1

1.50k

1%

C1 Nichicon

UPL1J330MEH, ESR = 0.35

Ω

C2 Nichicon

UPL1C221MPH, ESR = 0.16

Ω

D1 Motorola

MBR160

L1

Sumida

RCH106-221K, DCR = 0.78

Ω

PGND

1

7

4

2

5

3

Figure 3. 16V–24V to 12V/0.5A Buck Converter

Through Hole