Modeling of X-Ray Effects and Post-Irradiation Annealing in a Negative Low-Dropout Voltage Regulator

The LM2990T-5.0, an automotive low-dropout voltage regulator, was examined in two fields of X-ray radiation: 83-kVp and 300-kVp. After exposure to ionizing radiation, the tested devices were subjected to two annealing conditions: ten-year room-temperature annealing, followed by a sequence of one-week high-temperature annealing. Taking the experimental data on the maximum output current, minimum dropout voltage, quiescent current, and output voltage (as well as a SPICE simulation micromodel of this power-integrated circuit) into account, the influence of ionizing radiation and annealing on the following parameters were analyzed: leakage path resistance, forward emitter current gain of all bipolar junction transistors, and the power transistor emitter resistance. The simulation results indicated that variations in the forward emitter current gain recorded during the initial irradiation period had a negligible influence on the total radiation response and on the subsequent failure of the voltage regulator. Variations in the power transistor emitter resistance were more pronounced but did not cause integrated circuit failure. Leakage path resistance between the collector and emitter terminals of an NPN transistor in the control circuit proved to be the most influential parameter contributing to the observed shutdown. All of the irradiated circuits recovered and became functional again, after ten years of room-temperature annealing.