MIC4685WREV MICREL [Micrel Semiconductor], MIC4685WREV Datasheet - Page 10

• Manufacturer Part Number: MIC4685WREV
• Description: 3A SPAK SuperSwitcher Buck Regulator
• Manufacturer: MICREL [Micrel Semiconductor]

Functional Diagram
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
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.
Micrel, Inc.
January 2010
Enable
MIC4685
Oscillator
200kHz
Figure 1. Adjustable Regulator
EN
parator
Com-
Error
Amp
= 0V.
Shutdown
Bootstrap
Regulator
Charger
Thermal
Internal
Reset
1.235V
Bandgap
Reference
Current
Driver
Limit
10
IN
A higher feedback voltage increases the error amplifier
output
(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
inductor L1, to output capacitor C
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
Output Capacitor
External output capacitor C
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.
V
IN
SW
FB
voltage.
R1 R2
V
V
OUT
REF
=
=
=
1.235V
V
⎛
⎜
⎝
REF
V
V
R1
R2
OUT
REF
⎛
⎜
⎝
R2
R1
–1
A
⎞
⎟
⎠
+
V
C
OUT
1
OUT
⎞
⎟
⎠
higher
IN
OUT
, through external storage
provides stabilization and
error
OUT
OUT
.
amplifier
M9999-012610
and the load.
MIC4685
voltage