MIC4681
Micrel
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 MIC4681 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. See “Bode Plots” for additional information.
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.
Functional Description
The MIC4681 is a variable duty cycle switch-mode regulator
with an internal power switch. Refer to the block diagrams.
Supply Voltage
The MIC4681 operates from a +4V to +30V unregulated
input. Highest efficiency operation is from a supply voltage
around +12V. See the efficiency curves on page 6.
Enable/Shutdown
The shutdown (
SHDN
) input is TTL compatible. Ground the
input if unused. A logic-low enables the regulator. A logic-
high shuts down the internal regulator which reduces the
current to typically 35µA when V
SHDN
= V
IN
= 12V and 6µA
when V
SHDN
= 5V. See “Shutdown Input Behavior: Shutdown
Hysteresis.”
Feedback
Require an external resistive voltage divider from the output
voltage to ground, center tapped to the
FB
pin. See Figure 1b
for recommended resistor values.
Duty Cycle Control
A fixed-gain error amplifier compares the feedback signal
with a 1.23V 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
Applications Information
Adjustable Regulators
Adjustable regulators require a 1.23V feedback signal. Rec-
ommended voltage-divider resistor values for common out-
put voltages are included in Figure 1b.
For other voltages, the resistor values can be determined
using the following formulas:
V
IN
C
IN
SHUTDOWN
ENABLE
MIC4681BM
2
IN
SW
3
L1
R1
V
OUT
1
SHDN
GND
5–8
FB
4
C
OUT
D1
R2
R1
V
OUT
=
V
REF
+
1
R2
V
R1
=
R2
OUT
−
1
V
REF
V
REF
=
1.23V
Figure 1a. Adjustable Regulator Circuit
V
OUT
R1*
R2*
C
IN
D1
3A 40V Schottky
L1
68µH 2.0A
C
OUT
1.8V
3.01k 6.49k
2.5V
3.01k 2.94k
3.3V
3.01k 1.78k
5.0V
3.01k 976
Ω
6.0V
3.01k 787
Ω
* All resistors 1%
** shielded magnetics for low RFI applications
*** Vishay-Diode, Inc. (805) 446-4800
22µF 35V
Vishay Dale
593D226X035E2T
B340A Vishay-Diode, Inc.***
Coiltronics UP3B-680
or
or
SS36 General Semiconductor Sumida CDRH127-680MC**
220µF 10V
Vishay Dale
594D227X0010D2
Figure 1b. Recommended Components for Common Ouput Voltages
April 2005
11
MIC4681
MICREL [ MICREL SEMICONDUCTOR ]
