The XTR111 delivers the precise output current to the IS pin. The voltage at this pin is normally 1.4V less than V_VSP.

This output requires an external transistor (Q_EXT) that forms a cascode for the current output. The transistor must be rated for the maximum possible voltage on V_OUT and must dissipate the power generated by the current and the voltage across the transistor.

The gate drive (VG) can drive from close to the positive supply rail to 16V less than the positive supply voltage (V_VSP). Most modern MOSFETs accept a maximum V_GS of 20V. A protection clamp is only required if a large drain gate capacitance can pulse the gate beyond the rating of the MOSFET. Pulling the OD pin high disables the gate driver and closes a switch connecting an internal 3kΩ resistor from the VSP pin to the VG pin. This resistor discharges the gate of the external FET and closes the channel; see also Figure 6-6.

Figure 6-6 Equivalent Circuit for Gate Drive and Disable Switch

Table 6-1 lists some example devices in SO-compatible packages, but other devices can be used as well. Avoid external capacitance from IS. This capacitance can be compensated by adding additional capacitance from VG to IS; however, this compensation can slow down the output.

Select a drain-to-source breakdown voltage high enough for the application. Surge voltage protection can be required for negative overvoltages. For positive overvoltages, use a clamp diode to the 24V supply to help protect the FET from reversing.