MIC4684_10 Datasheet, PDF (9/16 Pages) Micrel Semiconductor – 2A High-Efficiency SuperSwitcher Buck Regulator

MIC4684
Block Diagrams
V_IN
IN
Enable
Bootstrap
Charger
Internal
Regulator
200kHz
Oscillator
Thermal
Shutdown
Current
Limit
Com-
parator
SW
Driver
Reset
FB
Error
Amp
1.235V
Bandgap
Reference
MIC4684
Adjustable Regulator
( ) V_OUT = V_REF
R1 _{+ 1}
R2
( ) R1= R2 VOUT - 1
V_REF
V_REF = 1.235V
V_OUT
C_OUT
R1
R2
Micrel, Inc.
Functional Description
The MIC4684 is a variable duty cycle switch-mode regula-
tor with an internal power switch. Refer to the above block
diagram.
Supply Voltage
The MIC4684 operates from a +4V to +30V (34V transient)
unregulated input. Highest efficiency operation is from a supply
voltage around +12V. See the efficiency curves on page 5.
Enable/Shutdown
The enable (EN) input is TTL compatible. Tie the input high
if unused. A logic-high enables the regulator. A logic-low
shuts down the internal regulator which reduces the current
to typically 150µA when V_EN = 0V.
Feedback
Fixed-voltage versions of the regulator have an internal resis-
tive 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 Table 1 and Table 2 for recommended resis-
tor values.
Duty Cycle Control
A fixed-gain error amplifier compares the feedback signal
with a 1.235V 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 invert-
ing 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 MIC4684 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.
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.
January 2010
9
M9999-012610