The TPS7A3901DSCR is a negative 200mA ultra-low-noise LDO from TI in VSON-8 (2x2mm), with an adjustable output from -0.2V to -5.5V. It is specifically designed for negative-rail applications in optical and RF systems. The NR (Noise Reduction) pin with an external capacitor filters the internal bandgap reference noise, achieving 3.8uVrms output noise (10Hz-100kHz) – among the lowest for any negative LDO. The PSRR is 72dB at 1kHz and 55dB at 10kHz, critical for rejecting switching converter ripple on negative rails. The FB pin regulates to -1.5V: VOUT = -1.5V x (1 + R1/R2) using resistors from OUT to FB to GND. Dropout voltage is 200mV at 200mA (e.g., -3.0V output requires VIN <= -3.2V). The EN pin (active HIGH, internal pull-up to IN) enables the output; pulling EN to GND shuts down the device with 0.3uA quiescent current. Unlike positive LDOs where the pass element is a PMOS, the negative LDO uses an NMOS pass transistor with the source connected to OUT and drain to IN. The VSON-8 package with exposed pad provides 60C/W thermal resistance. Applications include DAC/ADC negative reference supplies, op-amp negative rails, and optical transceiver bias.