The XTR200 integrates an output transistor capable of delivering the specified output current to a wide range of load resistances. However, in applications with high supply voltages, using an external transistor reduces the power dissipated in the XTR200. The Power Supply Recommendations section provides useful information on supply voltage and PCB temperature limitations when using the internal output transistor. Establish that the external transistor is rated for the maximum anticipated supply voltage and is capable of dissipating the power generated by the load current and the voltage drop across the transistor.

Figure 6-1 and Figure 6-2 display the current flow when using an external PNP or PMOS transistor with the XTR200. A portion of the load current flows through the internal 1kΩ resistor between the IS and VG pins, producing a voltage which turns on the external transistor. The voltage between the IS and VG pins is limited to approximately 2V by a clamp circuit represented by a Zener diode in the diagram. A small portion of the output load current still flows through the internal PMOS of the XTR200 but is recombined with the current through the external transistor. When using an external PNP transistor, the base current is recirculated through the internal PMOS (Q2) and does not degrade the accuracy of the output current.

All load current flows through the on-chip 50Ω resistor in the XTR200 which is used to measure output current and detect fault conditions. For this reason, using an external transistor does not change the transfer function of the XTR200 or increase the maximum output current. External transistors are protected from short-circuit faults by the same circuitry which protects the internal output PMOS.

Figure 6-1 Current Pathways for an External PNP Transistor

Figure 6-2 Current Pathways for an External PMOS Transistor