參數(shù)資料
型號(hào)： OP484
廠商： Analog Devices, Inc.
英文描述： Precision Rail-to-Rail Input & Output Operational Amplifiers(精密滿幅度輸入輸出運(yùn)放)
中文描述： 精密軌到軌輸入
文件頁(yè)數(shù)： 15/20頁(yè)
文件大小： 370K
代理商： OP484
REV. 0
–15–
OP184/OP284/OP484
10
0%
100
90
1nF LOAD
ONLY
SNUBBER
IN
CIRCUIT
2μs
50mv
50mv
μs
Figure 54. Overshoot and Ringing Is Reduced by Adding a
“ Snubber” Network in Parallel with the 1 nF Load
T able I. Snubber Networks for Large Capacitive Loads
Load Capacitance
(C
L
)
Snubber Network
(R
S
, C
S
)
1 nF
10 nF
100 nF
50
, 100 nF
20
, 1
μ
F
5
, 10
μ
F
A Low Dropout Regulator with Current Limiting
Many circuits require stable regulated voltages relatively close in
potential to an unregulated input source. T his “l(fā)ow dropout”
type of regulator is readily implemented with a rail-to-rail out-
put op amp such as the OP284, because the wide output swing
allows easy drive to a low saturation voltage pass device. Fur-
thermore, it is particularly useful when the op amp also enjoys a
rail-rail input feature, as this factor allows it to perform high-
side current sensing for positive rail current limiting. T ypical ex-
amples are voltages developed from 3 V to 9 V range system
sources, or anywhere where low dropout performance is required
for power efficiency. T he 4.5 V case here works from 5 V nomi-
nal sources, with worst-case levels down to 4.6 V or less.
Figure 55 shows such a regulator set up using an OP284 plus a
low R
DS(ON)
, P-channel MOSFET pass device. Part of the low
dropout performance of this circuit is provided by Q1, which
has a rating of 0.11
with a gate drive voltage of only 2.7 V.
T his relatively low gate drive threshold allows operation of the
regulator on supplies as low as 3 V without compromise to over-
all performance.
T he circuit’s main voltage control loop operation is provided by
U1B, half of the OP284. T his voltage control amplifier ampli-
fies the 2.5 V reference voltage produced by three terminal U2,
a REF192. T he regulated output voltage V
OUT
is then:
V
OUT
=
V
OUT
2
1
+
R
2
R
3
(
)
For the example here, a V
OUT
of 4.5 V with V
OUT 2
= 2.5 V re-
quires a U1B gain of 1.8 times, so R3 and R2 are chosen for a
ratio of 1.2:1, or 10.0 k
:8.06 k
(using closest 1% values).
Note that for the lowest V
OUT
dc error, R2
i
R3 should be main-
tained equal to R1 (as here), and the R2-R3 resistors should be
stable, close tolerance metal film types. T he table in Figure 55
summarizes R1-R3 values for some popular voltages. However,
note that in general the output can be anywhere between V
OUT 2
to the 12 V maximum rating of Q1.
While the low voltage saturation characteristic of Q1 is a key
part of the low dropout, another component is a low current
sense comparison threshold with good dc accuracy. Here, this
is provided by current sense amplifier U1A, which is provided a
20 mV reference from the 1.235 V AD589 reference diode D2
and the R7-R8 divider. When the product of the output current
and the R
S
value matches this voltage threshold, the current
control loop is activated, and U1A drives Q1’s gate through D1.
T his causes the overall circuit operation to enter current mode
control, with a current limit I
LIMIT
defined as:
I
LIMIT
=
V
R
(
D
2)
R
S
R
7
R
7
+
R
8
(
)
R9
27.4k
C4
0.1μF
C5
0.01μF
R11
1k
R6
4.99k
R8
301k
+V
S
R10
1k
R2
8.06k
C3
0.1μF
R
S
0.05
C6
10μF
3
2
1
8
4
R7
4.99k
U1A
OP284
D2
AD589
D1
1N4148
R5
22.1k
Q1
SI9433DY
R4
2.21k
6
5
7
C1
0.01μF
U1B
OP284
D3
1N4148
2
6
4
3
U2
REF192
C2
1μF
R1
4.53k
V
2
2.5V
R3
10k
5.0V
4.5V
3.3V
3.0V
4.99k
4.53k
2.43k
1.69k
10.0k
8.06k
3.24k
2.00k
10.0k
10.0k
10.0k
10.0k
V
OUT
R1
R2
R3
OUTPUT TABLE
V
S
>
V
OUT
+ 0.1V
OPTIONAL
ON/OFF CONTROL INPUT
CMOS HI (OR OPEN) = ON
LO = OFF
V
IN
COMMON
V
V
=
4.5V @ 350mA
(SEE TABLE)
V
OUT
COMMON
Figure 55. A Low Dropout Regulator with Current Limiting
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