MIC4575_09 Datasheet, PDF (5/15 Pages) Micrel Semiconductor – 200kHz Simple 1A Buck Regulator

Micrel, Inc.
Functional Description
The MIC4575 is a variable duty cycle switch-mode
regulator with an internal power switch. Refer to the
block diagrams.
Supply Voltage
The MIC4575 operates from a +4V to +24V 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 the1.23V 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.
MIC4575
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.
July 2009
5
M9999-072909