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 L9935 STEPPER-MOTOR DRIVER FOR LIGHTING APPLICATIONS IN CARS  Manually driven headlight levelling systems still use DC motors. These will soon be replaced, however, by stepper motors since automatic levelling is becoming more and more popular. ST's L9935 stepper motor driver includes two full bridge drivers with a current capability up to 1.2A DC and 2,5A peak. A sophisticated programmable current regulation system provides the appropriate phase current according to the needs of the application. With two external resistors the operating current range can be set for each full bridge independently. Full, half and quarter step operation is also supported so the circuit function can be optimized to the conditions of each application. According to EMC requirements, a slew rate control of the rectangular PWM output voltage is implemented to minimize high frequency radiation. The L9935 features short circuit protection of all outputs and many diagnostic functions such as error-flag for open load, overload and over temperature. A serial interface (SPI) has been integrated to communicate with the microcontroller.  Low current consumption of 40µA (typical) in standby mode allows direct battery operation. The application diagram shown on the right demonstrates how the circuit is implemented in an automatic levelling system, driven via the L9637 LIN-bus transceiver interface. Levelling data is provided by a sensor mounted on the shock absorber near the rear wheels. Depending on the load, the microcontroller moves the low beam to a save position. For high pressure gas discharge bulbs a dynamic levelling is required by law in many countries. On bumpy roads the levelling is permanently working and this requires high output power from the driver. For this reason, the circuit is housed in PowerSO20 package which provides very low thermal impedance. As a result, no external heatsink is required.  |
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