A feature that is becoming more common in modern operational amplifiers (op amps) is the ability to enable and disable the device. Texas Instruments (TI) currently has over 60 op amps with shutdown functionality and more on the way. This application note will provide an explanation of various shutdown parameters and design considerations for the TLV90xxS family of devices including the TLV900xS, TLV905xS and TLV906xS.

Amplifiers with shutdown functionality are well suited to applications that are battery powered and require portions of their system to turn off to save power. Examples of such systems include smoke detectors, PIR sensors, field transmitters and remote sensors. "Low power" amplifiers that do not have shutdown functionality typically have some tradeoffs, such as exchanging low quiescent current for very low bandwidth. On the other hand, amplifiers with shutdown functionality can offer low current consumption when not in use and greater bandwidth when in use. Despite requiring a bit more control, they can mitigate this power for performance tradeoff.

Amplifiers with shutdown functionality allow the user to "enable" and "disable," or "shutdown," the device even while the supply rails remain powered. When the amplifier is enabled, it functions as expected by amplifying the difference between the inverting and non-inverting inputs by the open loop gain (A_OL) and consuming the full amount of quiescent current (I_Q). When the amplifier is in shutdown, it consumes much less quiescent current and the output becomes high impedance.

Devices that are in a single or dual channel package usually have one SHDN pin per amplifier. Quad package devices typically come with two SHDN pins: one for controlling channels 1 and 2 and another for controlling channels 3 and 4. Figure 1-1 and Figure 1-2 show example pinouts for a dual channel shutdown device (TLV9062S) and a quad channel shutdown device (TLV9064S).

For the TLV90xxS family of products, the shutdown specifications are given in the “Electrical Characteristics” table of the device data sheet as shown for the TLV906xS in Figure 2-1 below. Not all TI op amps with shutdown functionality have the same information in their respective data sheets. However, most will specify the quiescent current when disabled, threshold voltages for the shutdown state, and enable/disable times. Definitions for the shutdown parameters follow in Table 2-1.

Figure 2-1 TLV906xS Electrical Characteristics Table Shutdown Specifications

Some shutdown specifications are similar but different from specifications for amplifiers in normal operation. For example, the previously defined quiescent current during shutdown, I_QSD, is similar to quiescent current, I_Q. However, the test condition for the TLV906xS specifies that I_QSD testing is done with all device channels disabled. As shown in Figure 2-1 above, the TLV906xS devices have a typical quiescent current of 0.5 µA while in the full shutdown state with all channels off. This is less than 0.1% of their typical I_Q per channel of 538 µA while enabled.

Similarly, Z_SHDN is reminiscent of the amplifier's open-loop output impedance while the amplifier is enabled (Z_O). Z_SHDN is specified in Figure 2-1 as a parallel combination of a resistance (R_SHDN) and a capacitance (C_SHDN). When plotted over frequency, it resembles the curve shown in Figure 2-2. At DC, this impedance is equal to the resistance given in the specification table, 10 GΩ in this case. As the frequency increases, the capacitance begins to dominate the response and reduces the overall output impedance. For the TLV906xS devices, this impedance falls to 200 kΩ at 100 kHz.

Figure 2-2 Output Impedance During Shutdown

Finally, the shutdown devices feature threshold voltages, specified by V_{SHDN_THR_HI} and V_{SHDN_THR_LO}, that define the enable and disable regions via the SHDN pin. Between these voltages is an undefined state where the amplifier may be on, off, or some combination of the two states. The threshold voltages for the TLV90xxS devices are defined relative to the supply voltage. However, other devices may give absolute voltages for a specific supply. Figure 2-3 illustrates the SHDN pin regions using the threshold values from the TLV906xS data sheet with the grey striped area representing the undefined state. The left bar shows the minimum and maximum threshold values from the data sheet and the right bar shows the typical threshold values. To ensure proper operation, the minimum and maximum thresholds should be met during design.

Figure 2-3 Shutdown Threshold Graphic