MIC4574
Micrel
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
The shutdown (
SHDN
) input is TTL compatible. Ground the
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
divider from the feedback (
FB
) pin. Connect
FB
directly to the
output line.
Adjustable versions require an external resistive voltage
divider from the output voltage to ground, connected from the
1.23V tap to
FB
.
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
IN,
through external storage inductor L1, to
output capacitor C
OUT
and the load. Energy is stored in the
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
OUT
.
Output Capacitor
External output capacitor C
OUT
provides stabilization and
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.
L1
3
For a mathematical approach to component selection and
circuit design, refer to Application Note 14.
SHDN
SW
7
3.3V/0.5A
8V to 24V
3
SHDN
SW
7
L1
100µH
5.0V/0.5A
100µH
1
6V to 24V
C1
47µF
35V
5
MIC4574-3.3BN
FB
V
IN
SGND PGND
2
4
D1
1N5819
C2
220µF
16V
5
C1
47µF
35V
MIC4574-5.0BN
FB
V
IN
SGND PGND
2
4
1
D1
1N5819
C2
220µF
16V
C1
C2
D1
L1
Nichicon
Nichicon
Motorola
Sumida
UPL1V470MEH, ESR = 0.34Ω
UPL1C221MPH, ESR = 0.16Ω
1N5819
RCH875-101K, DCR = 0.28Ω
C1
C2
D1
L1
Nichicon
Nichicon
Motorola
Sumida
UPL1J470MEH, ESR = 0.34Ω
UPL1C221MPH, ESR = 0.16Ω
1N5819
RCH875-101K, DCR = 0.28Ω
Figure 1. 6V–24V to 3.3V/0.5A Buck Converter
Through Hole
3
Figure 2. 8V–24V to 5V/0.5A Buck Converter
Through Hole
SW
FB
PGND
4
7
L1
220µH
R2
13.0k
1%
R1
1.50k
1%
12V/0.5A
SHDN
V
IN
SGND
2
16V to 24V
C1
33µF
63V
5
MIC4574BN
1
MBR160
C2
220µF
16V
C1
C2
D1
L1
Nichicon
Nichicon
Motorola
Sumida
UPL1J330MEH, ESR = 0.35Ω
UPL1C221MPH, ESR = 0.16Ω
MBR160
RCH106-221K, DCR = 0.78Ω
Figure 3. 16V–24V to 12V/0.5A Buck Converter
Through Hole
4-96
April 1998•
MICREL [ MICREL SEMICONDUCTOR ]
