Micrel, Inc.
MIC4680
Functional Description
The MIC4680 is a variable duty cycle switch-mode
regulator with an internal power switch. Refer to the
block diagrams.
Supply Voltage
The MIC4680 operates from a +4V to +34V unregulated
input. Highest efficiency operation is from a supply
voltage around +15V. See the efficiency curves.
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 1.5µA when V
SHDN
= V
IN
= 12V and 30µA when V
SHDN
= 5V. See “Shutdown Input
Behavior: Shutdown Hysteresis.”
Feedback
Fixed-voltage versions of the regulator have an internal
resistive divider from the feedback (FB) pin. Connect FB
directly to the output voltage.
Adjustable versions require an external resistive voltage
divider from the output voltage to ground, center tapped
to the FB pin. See Figure 6b for recommended resistor
values
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. The MIC4680 uses a voltage-
mode control architecture.
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. See “Bode Plots” for additional
information.
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.
March 2008
9
M9999-032808
MICREL [ MICREL SEMICONDUCTOR ]
