MIC4576 Micrel, Inc.
MIC4576 6 March 2006
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
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 comparator 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 in-
formation, 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.
Note 2: Surface-mount component.
C1
470
µF
35V
D1
1N5822
C2
1000µF
16V
L1
33
µH
GND
FB
SW
MIC4576-3.3BT
SHDN
6V to 24V
3.3V/3A
V
IN
C1 Nichicon UPL1V471MPH, ESR = 0.046Ω
C2 Nichicon UPL1C221MPH, ESR = 0.047Ω
D1 Motorola 1N5822
L1 Coiltronics PL52C-33-1000, DCR = 0.036Ω
L1 Bi HM77-30004, DCR = 0.045Ω, Note 2
3
4
25
1
Figure 1. 6V–24V to 3.3V/3A Buck Converter
Through Hole
C1
470
µF
63V
D1
MBR360
C2
1000µF
16V
L1
33
µH
GND
FB
SW
MIC4576-3.3BT
SHDN
6V to 36V
3.3V/3A
V
IN
C1 Nichicon UPL1J471MRH, ESR = 0.039Ω
C2 Nichicon UPL1C102MPH, ESR = 0.047Ω
D1 Motorola MBR360
L1 Coiltronics PL52C-33-1000 DCR = 0.036
L1 Bi HM77-30004, DCR = 0.045Ω, Note 2
3
4
25
1
Figure 2. 6V–36V to 3.3V/3A Buck Converter
Through Hole
