Micrel, Inc.
MIC4685
Functional Diagram
V
IN
IN
Bootstrap
Charger
Enable
Internal
Regulator
⎛
R1
⎞
V
OUT
=
V
REF
⎜
+
1
⎟
⎝
R2
⎠
Current
Limit
⎛
V
⎞
R1
=
R2
⎜
OUT
–1
⎟
⎝
V
REF
⎠
V
REF
=
1.235V
200kHz
Oscillator
Thermal
Shutdown
Com-
parator
Driver
Reset
R1
FB
Error
Amp
MIC4685
1.235V
Bandgap
Reference
R2
SW
V
OUT
C
OUT
Figure 1. Adjustable Regulator
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
In the adjustable version, 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 comparator output. A lower feedback voltage
increases the duty cycle. The MIC4685 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
10
M9999-012610
MICREL [ MICREL SEMICONDUCTOR ]
