TPS26610 and TPS26613 devices allow a bipolar current limited conduction through the device even when the external +Vs/–Vs supplies are not there. When the external supply is not there, the device switches to loop power mode and derives its operating power from the 4–20-mA or ±20-mA current loop. This feature enables the field installation engineer to check the wiring of the whole current loop system by passing a test current through the current loop without actually powering on the system. This feature also helps in design of safety critical redundant systems with two redundant measurements for the same current loop. In case power is not available in one system, a second system connected in the loop is still be able to read the current information because the loop is not broken. During loop testing without ±Vs supply, the device has a voltage drop of V_{(IN-OUT)no_Vs}, the current through device is limited to I_{OL_noVs}. During loop testing, the device draws a current of I_{OL_noVs} from IN pin. See the Electrical Characteristics in Specifications for V_{(IN-OUT)no_Vs}, I_{qno_Vs} and I_{OL_noVs}.

The device provides thermal protection during loop testing, if the power dissipation in device increases above 500 mW (typical), the devices turns off internal FET for short durations to limit the power dissipation. Figure 8-16 and Figure 8-17 illustrate the thermal protection during loop testing.