• Linear Li‑ion charger delivers 400 mA constant current from a 5 V supply, then switches to 4.217 V constant voltage. • Dual‑loop control uses a PNP pass transistor, with inner current loop and outer voltage loop for precise regulation. • Built with only half of an LM324 op‑amp, the design keeps component count low and cost‑effective. • The charger’s 1.25 V reference, 115 kΩ and 10 kΩ resistors, and 0.25 Ω sense resistor generate a 0.10 V current reference. • LTSpice simulation and bench testing confirm reliable CC‑CV behavior, with a 1N4007 diode ensuring safe operation. • Ideal for educational labs, embedded products, and DIY bench chargers, offering transparent, first‑principles design.
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