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.
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.
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 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
When the internal switch is turned OFF, the collapse of the
magnetic field in L1 forces current to flow through fast
current to typically 35µA when V
= V = 12V and 6µA
recovery diode D1, charging C
.
SHDN
IN
OUT
whenV
=5V. See“ShutdownInputBehavior:Shutdown
SHDN
Output Capacitor
Hysteresis.”
External output capacitor C
provides stabilization and
OUT
Feedback
reduces ripple. See “Bode Plots” for additional information.
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.
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.
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.
MIC4681BM
VIN
CIN
L1
VOUT
2
1
3
4
IN
SW
For other voltages, the resistor values can be determined
using the following formulas:
R1
R2
SHUTDOWN
ENABLE
SHDN
FB
COUT
D1
GND
5–8
R1
V
= V
+1
OUT
REF
R2
V
Figure 1a. Adjustable Regulator Circuit
OUT
R1= R2
−1
V
REF
V
= 1.23V
REF
V
R1*
R2*
C
D1
L1
C
OUT
OUT
IN
1.8V 3.01k 6.49k
2.5V 3.01k 2.94k
3.3V 3.01k 1.78k
5.0V 3.01k 976Ω
6.0V 3.01k 787Ω
3A 40V Schottky
68µH 2.0A
22µF 35V
Vishay Dale
593D226X035E2T
220µF 10V
Vishay Dale
594D227X0010D2
B340A Vishay-Diode, Inc.***
or
SS36 General Semiconductor Sumida CDRH127-680MC**
Coiltronics UP3B-680
or
*
All resistors 1%
** shielded magnetics for low RFI applications
*** Vishay-Diode, Inc. (805) 446-4800
Figure 1b. Recommended Components for Common Ouput Voltages
February 2001
11
MIC4681
MICREL [ MICREL SEMICONDUCTOR ]
