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

1

Ω

9 µA

3 mA

770

Ω

100 k

Ω

0.9 µA

300 µA

7.8 k

Ω

1M

Ω

99 nA

55 µA

70.8 k

Ω

The smallest load resistor that the LPV531 can drive when in

low power mode is 70.8 k

Ω, as shown in Table 2. When

driving smaller loads, such as the 10 k

Ω 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

Ω load then it is not recommended to use the

LPV531 in low power mode.

I

The I

entire internal bias condition. Therefore, the I

sensitive to parasitic signal coupling. In order to protect the

I

PCB layout such that there is as little coupling between the

I

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

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

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

ternal Miller compensation capacitor (C

The non-

dominant pole is formed by the transconductance of the

output stage (g

the output of the LPV531 (C

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

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