Although the INA199-Q1 series does not have a shutdown pin, the low power consumption of the device allows the output of a logic gate or transistor switch to power the INA199-Q1. This gate or switch turns on and turns off the INA199-Q1 power-supply quiescent current.

However, in current-shunt monitoring applications, there is also a concern for how much current is drained from the shunt circuit in shutdown conditions. Evaluating this current drain involves considering the simplified schematic of the INA199-Q1 in shutdown mode as shown in Figure 9-3.

1-MΩ paths from shunt inputs to the reference and the INA199-Q1 outputs.

Figure 9-3 Basic Circuit for Shutting Down the INA199-Q1 With a Grounded Reference

There is typically slightly more than a 1-MΩ impedance (from the combination of the 1-MΩ feedback and 5-kΩ input resistors) from each input of the INA199-Q1 to the OUT pin and to the REF pin. The amount of current flowing through these pins depends on the respective ultimate connection. For example, if the REF pin is grounded, the calculation of the effect of the 1-MΩ impedance from the shunt to ground is straightforward. However, if the reference or operational amplifier is powered when the INA199-Q1 is shut down, then the calculation is direct; instead of assuming a 1-MΩ impedance to ground, assume a 1-MΩ impedance to the reference voltage. If the reference or operational amplifier is also shut down, some knowledge of the reference or operational amplifier output impedance under shutdown conditions is required. For instance, if the reference source functions as an open circuit when not powered, little or no current flows through the 1-MΩ path.

Regarding the 1-MΩ path to the output pin, the output stage of a disabled INA199-Q1 does constitute a good path to ground. Consequently, this current is directly proportional to a shunt common-mode voltage applied across a 1-MΩ resistor.