MIC4685
Micrel
Block Diagrams
VIN
IN
Bootstrap
Charger
Enable
Internal
Regulator
R1
R2
V
= V
+1
OUT
REF
V
OUT
R1= R2
−1
200kHz
Oscillator
Thermal
Shutdown
Current
Limit
V
REF
V
= 1.235V
REF
Com-
parator
VOUT
SW
FB
Driver
COUT
Reset
R1
R2
Error
Amp
1.235V
Bandgap
Reference
MIC4685
FIgure 1. Adjustable Regulator
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.
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.
Output Switching
When the internal switch is ON, an increasing current flows
from the supply V through external storage inductor L1, to
IN,
Enable/Shutdown
output capacitor C
and the load. Energy is stored in the
OUT
inductor as the current increases with time.
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
When the internal switch is turned OFF, the collapse of the
magnetic field in L1 forces current to flow through fast
to typically 150µA when V = 0V.
recovery diode D1, charging C
.
EN
OUT
Feedback
Output Capacitor
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.
External output capacitor C
reduces ripple.
provides stabilization and
OUT
Return Paths
Duty Cycle Control
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.
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.
September 2002
9
MIC4685
MICREL [ MICREL SEMICONDUCTOR ]
