JAJSF33B August 2017 – July 2018 THS3491
The THS3491 features a power-down (PD) pin that lowers the quiescent current from 16.7 mA down to 750 µA, which is designed to reduce system power.
The power-down pin of the amplifier defaults to 2 V below the positive supply voltage in the absence of an externally applied voltage, which places the amplifier in the power-on mode of operation. To turn off the amplifier in an effort to conserve power, the power-down pin can be pulled low. The PD pin threshold voltages are specified with respect to the REF pin voltage. The threshold voltages for power on and power down are relative to the REF pin and are shown in the Electrical Characteristics: VS = ±15 V and Electrical Characteristics: VS = ±7.5 V tables. Above the enable threshold voltage, the device is on. Below the disable threshold voltage, the device is off. The behavior is not specified between these threshold voltages.
This power-down functionality helps the amplifier consume less power in power-down mode. Power-down mode is not intended to provide a high-impedance output. The power-down functionality is not intended for use as a tri-state bus driver. In power-down mode, the impedance looking back into the output of the amplifier is dominated by the feedback and gain-setting resistors, but the output impedance of the device varies depending on the voltage applied to the outputs.
As with most current-feedback amplifiers, the internal architecture places limitations on the system in power-down mode. The most common limitation is that the amplifier turns on if there is a ±1 V or greater difference between the two input nodes (VIN+ and VIN–) of the amplifier. If this difference exceeds ±1 V, the amplifier creates an output voltage equal to approximately [(VIN+ – VIN–) –0.7 V] × gain. Conversely if a voltage is applied to the output while in power-down mode, the VIN– node voltage is equal to VO(applied) × RG / (RF + RG). For low-gain configurations and a large applied voltage at the output, the amplifier may turn on because of the aforementioned behavior.
The time delays associated with turning the device on and off are specified as the time it takes for the amplifier to reach 10% or 90% of the final output voltage. The time delays are in nanoseconds during power on and microseconds during power off because the amplifier moves out of linear operating mode for power-off conditions.