Driver chip simplifies automotive LED headlights

May 12, 2015 // By Christoph Hammerschmidt
The AEC Q100 qualified DC-DC converter controller A6271 from Allegro MicroSystems Europe provides a programmable constant-current output for driving high-power LEDs in series in automotive applications.

The device is based on a programmable fixed-frequency peak-current mode control architecture, and the DC-DC converter can be configured in a variety of different switching configurations including boost, buck-boost, SEPIC (single-ended primary inductor convertor) and buck (ground referenced switch). It is designed for automotive exterior lighting applications including high-power LED lighting systems, fog lights, reversing lights, daytime running lights, position lights and headlights.

The A6271 provides a cost-effective solution using an external logic-level MOSFET and minimum additional external components. The maximum LED current is set with a single external sense resistor and can be accurately modulated using a current reference input for analogue control.

External PWM dimming is possible via the PWM input, which also provides a shutdown mode. As an alternative, an internal PWM dimming circuit can be used by programming the PWM input and duty-cycle programming pins. Either PWM scheme controls the PWM output, which drives an external p-channel MOSFET connected in series with the LED string. This MOSFET is also used to isolate the load during certain fault conditions, including output shorts to ground.

During the design of the chip, particular emphasis has been put on minimising electromagnetic emissions through distributed decoupling and an externally programmable frequency dither circuit configured for the EMI specification CISPR 25. It is also possible to program the fundamental switching frequency below 150 kHz, where most EMI standards begin.

The device has a comprehensive set of integrated protection features to protect the IC, the LED driver system, and the LED string against faults. Fixed-output overvoltage protection ensures that no maximum voltage rating violations occur, even under a single point failure of the programmable output overvoltage protection circuit. Other protection features include LED overload (boost, SEPIC), output undervoltage (buck or buck-boost), input supply undervoltage, 5 V regulator output undervoltage, high-side supply (PWM PMOS) undervoltage, and thermal protection.

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