7.3.4 Power-Down Operation

The OPA837 includes a power-down mode in the 6-pin SOT23-6 package. Under logic control, the amplifier can switch from normal operation to a standby current of less than 10 µA. When the PD pin is connected high, the amplifier is active. Connecting the PD pin low disables the amplifier and places the output in a high-impedance state. When the amplifier is configured as a unity-gain buffer, the output stage is in a high DC-impedance state. A new feature in the OPA837 is a switch from the external inverting input pin to the internal active transistors. This switch operates with the disable pin function to open up the connection to the internal devices when powered down. Operating in unity gain provides a high-impedance voltage into both the output and inverting input pins. This feature allows direct active multiplexer operation to be implemented; see Figure 87. The TIDA-01565 Wired OR MUX and PGA Reference Design demonstrates the use of the OPAx837 in wired-OR multiplexer and programmable gain amplifier applications. When disabled, the internal input devices on the inverting input approximately follow the noninverting input on the other side of the open switch through the back-to-back protection diodes across the inputs. When powered up, these diodes (two in each direction) act to limit overdrive currents into the active transistors.

The PD pin must be actively driven high or low and must not be left floating. If the power-down mode is not used, PD must be tied to the positive supply rail.

PD logic states are referenced relatively low to the negative supply rail, V_S–. When the op amp is powered from a single-supply and ground, and the disable line is driven from logic devices with similar V_DD voltages to the op amp, the disable operation does not require any special consideration. The OPA837 is specified to be off with PD driven to within 0.55 V of the negative supply and specified to be on when driven more than 1.5 V above the negative supply. Slight hysteresis is provided around a nominal 1-V switch point; see Figure 58. When the op amp is powered from a split supply with V_S– below ground, a level shift logic swing below ground is required to operate the disable function.