MOSFET driver chip replaces relays in safety-critical applications

June 02, 2014 // By Christoph Hammerschmidt
The A6861 from Allegro Microsystems is an N-channel MOSFET driver IC that safely isolates three-phase motors in safety-critical automotive applications. The device is ideal to replace relays to switch the motor phases in applications like electronic power steering or electric brakes.

Allegro's A2-SIL product family has been optimised to reach even the most demanding ASIL ratings easily. It is customised to replace conventional electro-mechanic relays in safety-critical applications. On top of this, the driver IC offers an output pin that can be used to drive external inverse-polarity protection circuit.

To keep the power MOSFETs in 'On' state across the entire supply voltage range and during high phase voltage slew rates, the A6861 is equipped with three independent floating gate driver outputs. Very few external components suffice to build a circuit that separates the load from the supply even under high currents.

An integrated charge pump generates the voltage required to keep the high side power MOSFETs in 'On' state, provided the phase voltage is in the range of the battery voltage. The three gate drivers can be controlled separately through logic inputs. In a typical circuit the MOSFETs are switched to 'On' within 8µs and to 'Off' within 1µs. An undervoltage detection circuit continuously monitors if the charge pump voltage is sufficient to keep the MOSFETs in a safe, conducting status.

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