Motor drive boosts vibrator efficiency

Single-phase brushless motor driver IC cuts power consumption and boosts reliability for handset vibration motors.

The Melexis MLX90283 is a single-phase brushless vibration motor driver IC designed specifically to reduce the power budget for vibrating alerts in cellular handsets.

The device can also be used in applications for force feedback or haptic interfaces in gaming systems.

The MLX90283 integrates a Hall-effect sensor, logic and power driver circuitry into a low profile ultra-thin QFN package and introduces the new proprietary Active Start design to improve single-phase vibration motor startup reliability.

The operating voltage range from 1.8 to 3.6V is ideal for battery-powered applications.

Vibration motors are mostly battery powered, and are subject to stringent input power requirements.

Further reduction of current consumption for battery-saving is of primary interest but is challenged by the need for large vibrations, meaning high rotation speed.

The MLX90283 successfully applies the freewheel principle to vibration motors by exploiting single-phase motor characteristics.

In normal rotation, a conventional driver IC supplies the motor even when the rectified sinusoidal angular torque approaches zero.

This conflicts with power management objectives as the motor consumes unneeded energy.

In contrast, the MLX90283 switches off its outputs when the torque approaches zero and delays the next switchover using a freewheel delay of 92us (typical).

As the device activates the freewheel when the torque is close to zero, the rotation speed is unchanged.

The MLX90283 freewheel intelligent driving enables the reduction of the average motor current consumption up to 10mA, while keeping the same rotation speed.

This is the key to the high efficiency levels achieved with the MLX90283.

Another big issue in single-phase brushless motors is related to incorrect starting position - referred to as the deadpoint issue.

These simple DC brushless motors often rely on the use of a mechanical detent scheme called a cogging plate to prevent the rotor from stopping in a deadpoint.

Thinner and smaller motor designs like those needed for vibration motors inevitably reduce the cogging plate efficiency.

This can lead to unreliable startup or a "locked rotor" condition and could prevent the vibrating alert from functioning.

The MLX90283 detects a locked state within 110ms (typical) by sensing the rotor magnetic field.

Once this detection period has elapsed, the device enters Active Start mode and applies a 55Hz frequency (typical) to the motor coil.

This electrical oscillation is converted into mechanical shaking, producing swift movement of the rotor in forward and reverse direction.

Single-phase vibration motors are generally optimised for one rotation direction.

Reverse rotation reduces speed and raise current consumption.

The MLX90283 Active Start shakes the rotor in the forward direction seven times more than in the reverse direction (typically 15.9ms forward and 2.2ms reverse).

This ratio is the optimum to give most reliable start up in forward direction.

The MLX90283 preserves positive torque with this seven-eighths duty cycle, and so favours the initial rotation direction and maintains the designed motor performance.

The device exits the Active Start mode once rotation is detected by the Hall sensor.

Melexis Active Start is an innovative technology exclusively developed and implemented at Melexis that solves the deadpoint issue while favouring initial motor rotation direction.

The MLX90283 device is provided in a chip scale package, the smallest packaged IC configuration available.

The CSP saves valuable PCB and housing space allowing smaller integration.

The footprint of the new ultra-thin QFN (quad flat no leads) package is only 3mm2 (1.5 x 2mm) with 0.43mm maximum thickness.

This is particularly important for vibration motors fitted in mobile phone and consumer application where the demand is unceasingly for miniaturisation and ultraslim designs.

The MLX90283 is priced at Eur 0.25 in 5000-unit quantities.

Engineering samples are available on request.

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