MIC4681_05 MICREL [Micrel Semiconductor], MIC4681_05 Datasheet - Page 11

• Manufacturer Part Number: MIC4681_05
• Description: 2A-Peak SuperSwitcher SOP-8 Buck Regulator
• Manufacturer: MICREL [Micrel Semiconductor]

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 (
input if unused. A logic-low enables the regulator. A logic-
high shuts down the internal regulator which reduces the
current to typically 35 A when V
when V
Hysteresis.”
Feedback
Require an external resistive voltage divider from the output
voltage to ground, center tapped to the
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:
April 2005
MIC4681
R1 R2
V
V
*
* *
*
V
1
2
3
5
6
* *
SHDN
OUT
REF
8 .
5 .
3 .
0 .
0 .
O
A
s
V
U
h
l l
V
V
V
V
V
s i
T
e i
e r
h
d l
a
s
e
- y
s i
3
3
3
3
3
d
= 5V. See “Shutdown Input Behavior: Shutdown
R
D
o t
0 .
0 .
0 .
0 .
0 .
1.23V
m
V
o i
s r
* 1
1
1
1
1
1
a
REF
d
V
V
SHDN
k
k
k
k
k
1
g
, e
OUT
%
n
REF
e
n I
6
2
1
9
7
i t
. c
R
s c
4 .
9 .
7
8
7 .
* 2
6
7
8 (
R2
9
4
8
f
R1
r o
) input is TTL compatible. Ground the
0
k
k
k
) 5
o l
1
w
4
Figure 1b. Recommended Components for Common Ouput Voltages
4
R
6
1
4 -
I F
5
8
9
a
0
3
p
0
V
D
p
2
s i
c i l
2
2
h
t a
2
µ
SHDN
a
o i
C
6
F
y
X
n
N I
3
s
D
0
5
3
a
V
5
e l
FB
= V
E
2
T
pin. See Figure 1b
IN
= 12V and 6 A
S
B
S
3
3
4
6
0
A
3
G
A
e
V
n
4
s i
r e
0
h
V
l a
a
D
r o
- y
S
S
1
c
D
e
h
11
o i
m
o
d
t t
c i
, e
y k
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
output capacitor C
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. 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.
o
SHUTDOWN
n
n I
d
ENABLE
c
u
* .
t c
* *
r o
Figure 1a. Adjustable Regulator Circuit
S
V
u
IN
m
C
C
d i
o
IN
t l i
a
o r
C
6
i n
8
D
IN,
µ
2
1
s c
R
OUT
H
L
r o
H
IN
SHDN
1
through external storage inductor L1, to
U
2
1
MIC4681BM
0 .
2
P
7
and the load. Energy is stored in the
3
A
6 -
GND
B
6 -
8
5–8
0
8
M
0
OUT
C
SW
FB
* *
OUT
3
4
provides stabilization and
.
5
9
D1
4
V
2
D
s i
2
2
L1
0
h
2
C
µ
a
7
O
F
y
X
U
T
D
0
1
0
0
a
V
1
e l
0
R1
R2
D
2
MIC4681
Micrel
V
C
OUT
OUT