Datasheet

or
R
EXT
=
-11 k:
0.07
R
LOAD,MIN
R
LOAD,MIN
=
0.07
R
EXT
+ 11 k:
LPV531
SNOSAK5B –MARCH 2006–REVISED MARCH 2013 www.ti.com
(5)
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
I
SEL
I
SC
R
LOAD
1Ω 9 µA 3 mA 770Ω
100 kΩ 0.9 µA 300 µA 7.8 kΩ
1 MΩ 99 nA 55 µA 70.8 kΩ
The smallest load resistor that the LPV531 can drive when in low power mode is 70.8 kΩ, as shown in Table 2.
When driving smaller loads, such as the 10 kΩ load resistor used in the Electrical Characteristics tables
specification, the output swing in the low power mode is limited. If the application requires a 10 kΩ 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 39 shows the schematic of an inverting AC coupled amplifier using the LPV531 with the I
SEL
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 shutdown pin. Both of these methods have the
problem of allowing 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 depending 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 capacitors. 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.
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