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參數(shù)資料
| 型號： | LPV531 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Programmable Micropower CMOS Input, Rail-to-Rail Output Operational Amplifier |
| 中文描述： | 可編程微CMOS輸入，軌到軌輸出運算放大器 |
| 文件頁數(shù)： | 14/17頁 |
| 文件大?。?/td> | 955K |
| 代理商： | LPV531 |
Application Information
(Continued)
For the characterized power modes these equations lead to
the minimum values in
Table 2
below.
TABLE 2. Minimum Values for Characterized
Power Modes
R
EXT
1
100 k
1 M
I
SEL
9 μA
0.9 μA
99 nA
I
SC
3 mA
300 μA
55 μA
R
LOAD
770
7.8 k
70.8 k
The smallest load resistor that the LPV531 can drive when in
low power mode is 70.8 k
, as shown in
Table 2
. When
driving smaller loads, such as the 10 k
load resistor used in
the Electrical Characteristics table specification, the output
swing in the low power mode is limited. If the application
requires a 10 k
load then it is not recommended to use the
LPV531 in low power mode.
I
SEL
SENSITIVITY
The I
SEL
pin is a current reference that directly affects the
entire internal bias condition. Therefore, the I
SEL
pin is very
sensitive to parasitic signal coupling. In order to protect the
I
SEL
pin from unwanted distortion, it is important to route the
PCB layout such that there is as little coupling between the
I
SEL
pin and the output or other signal traces as possible.
Typical Application
AC COUPLED CIRCUITS
The programmable power mode makes the LPV531 ideal for
AC coupled circuits where the circuit needs to be kept active
to maintain a quiescent charge on the coupling capacitors
with minimal power consumption.
Figure 5
shows the sche-
matic of an inverting AC coupled amplifier using the LPV531
with the I
pin controlled by I/O ports of a microcontroller.
The advantage of the low power active mode for AC coupled
amplifiers is the elimination of the time needed to re-
establish a quiescent operating point when the amplifier is
switched to full power mode.
When an amplifier without a low power active mode is used
in low power applications, there are two ways to minimize
power consumption. The first method turns off the amplifier
by switching off power to the op amp using a transistor
switch. The second method uses an amplifier with a shut-
down pin. Both of these methods have the problem of allow-
ing the coupling capacitors, C
1
and C
2
to discharge the
quiescent DC voltage stored on them when in the shutdown
state. When the amplifier is turned on again, the quiescent
DC voltages must reestablish themselves. During this time,
the amplifier’s output is not usable because the output signal
is a mixture of the amplified input signal and the charging
voltage on the coupling capacitors. The settling time can
range from a several milliseconds to several seconds de-
pending on the resistor and capacitor values.
When the LPV531 is placed into the low power mode, the
power consumption is minimal but the amplifier is active to
maintain the quiescent DC voltage on the coupling capaci-
tors. The transition back to the operational high power mode
is fast, within a few hundred nanoseconds. The active low
power mode of the LPV531 separates two critical aspects of
a low power AC amplifier design. The values of the gain
resistors, bias resistors, and coupling capacitors can be
chosen independently of the turn-on and stabilization time.
PROGRAMMABLE POWER LEVELS AND THE
EFFECTS OF STABILITY COMPENSATION METHODS
USING EXTERNAL COMPONENTS
In some op amp application circuits, external capacitors are
used to improve the stability of the feedback loop around the
amplifier. When using the programmable power level feature
of the LPV531 such stability improvement methods may not
work. This is related to the internal frequency compensation
method applied inside the LPV531.
Figure 6
shows the bode plot of the frequency response of
the LPV531. The gain-bandwidth product is determined by
the transconductance of the input stage (g
m,in
) and the in-
ternal Miller compensation capacitor (C
). The non-
dominant pole is formed by the transconductance of the
output stage (g
m,out
) and the load capacitance connected to
the output of the LPV531 (C
l
). The frequency response
crosses the frequency axis with a single-pole slope
(20 dB/decade). This ensures the stability of feedback loops
formed around the LPV531.
20132334
FIGURE 5. Inverting AC Coupled Application
20132371
FIGURE 6. Bode Plot of the Frequency Response
L
www.national.com
14
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