Constant Current Load Circuit to Maintain 400mA for Power Supply Testing

The discussion focuses on designing a constant current load circuit to maintain approximately 400mA for power supply testing, ensuring the device under test (DUT) voltage remains stable around 1.5V (range 1.425–1.575V). The load should activate to supplement current when the DUT draws less than 400mA and deactivate when the DUT current demand increases, maintaining a combined constant current. Key challenges include minimizing voltage variation at the DUT, managing supply impedance, and avoiding interference with the DUT ground. A linear circuit approach is preferred to maintain consistent voltage regardless of DUT current fluctuations. Proposed solutions include using a PNP transistor with a zener diode for voltage reference, careful selection of sense resistor values, and consideration of transistor gain and supply impedance. Simulation results indicate voltage variations around 16–18mV for 100mA current changes, corresponding to an effective impedance near 180mΩ, which may be acceptable depending on tolerance. Faster switching introduces voltage spikes, which are undesirable, and adding capacitors on the DUT is not an option due to testing constraints. Alternative simpler solutions suggested include using an LM317 adjustable regulator with a 3Ω resistor to set current, requiring sufficient voltage headroom, and an electronic rheostat circuit for load current independent of applied voltage, with high-side current monitoring via an INA128 instrumentation amplifier. The discussion emphasizes the need for clear voltage variation or impedance specifications to optimize the circuit design.
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