This project is good solution if you want charge your laptop inside your car. ASUS Eee 12v Car Charger circuit based on LM2576 “Simple Switcher” from National Semiconductor. The IC has lower drop-out voltage (~2V) which is important when powering the device from a car or 12v SLA battery. The charger uses PE92108KNL Inductor from Pulse Engineering for low switching frequency needs.
To prevents unstable operation of the regulator at lower input voltages and helps prevent accidental flattening of the supply battery, the circuit includes a low voltage cut-out based on a 9.1v Zener diode and BC337 transistor. It will shut down the regulator if the input voltage is below 11.5V. Substituting this transistor for similar type may affect the cut-out voltage; the Vbe of the transistor should be 1.2v. Don’t forget to use high temperature and low ESR capacitors for more stable operation and better efficiency of the charger. download circuit.