mic4574 Micrel Semiconductor, mic4574 Datasheet - Page 5

• Manufacturer Part Number: mic4574
• Description: Mic4574 200khz Simple 0.5a Buck Regulator
• Manufacturer: Micrel Semiconductor

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 (
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 (
output line.
Adjustable versions require an external resistive voltage
divider from the output voltage to ground, connected from the
1.23V tap to
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.
June 2005
MIC4574
FB
.
SHDN
) input is TTL compatible. Ground the
FB
) pin. Connect
FB
directly to the
5
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.
IN,
OUT
through external storage inductor L1, to
and the load. Energy is stored in the
OUT
OUT
provides stabilization and
.
M9999-062105
Micrel, Inc.