MIC4685 Datasheet, PDF (9/15 Pages) Micrel Semiconductor – 3A SPAK SuperSwitcher™ Buck Regulator

MIC4685
Block Diagrams
V_IN
IN
Enable
Bootstrap
Charger
Internal
Regulator
200kHz
Oscillator
Thermal
Shutdown
Current
Limit
Com-
parator
Reset
Driver
Error
Amp
MIC4685
1.235V
Bandgap
Reference
V_OUT
=
V_REF

R1
R2
+ 1^{}
R1
=
R2


V_OUT
V_REF

− 1_
V_REF = 1.235V
SW
V_OUT
C_OUT
R1
FB
R2
FIgure 1. Adjustable Regulator
Micrel
Functional Description
The MIC4685 is a variable duty cycle switch-mode regulator
with an internal power switch. Refer to the above block
diagram.
Supply Voltage
The MIC4685 operates from a +4V to +30V (34V transient)
unregulated input. Highest efficiency operation is from a
supply voltage around +12V. See the efficiency curves in the
Typical Characteristics section 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
An external resistive voltage divider is required from the
output voltage to ground, center tapped to the FB pin. See
Table 1 and Table 2 for recommended resistor 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
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. The MIC4685 uses a voltage-mode control architec-
ture.
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.
September 2002
9
MIC4685