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Vibration motor driver IC uses Freewheel principle

Posted: 13 Sep 2007 ?? ?Print Version ?Bookmark and Share

Keywords:Vibration motor driver? Freewheel prnciple?


Ringtones are a common way to personalize one's cellphone, PDA or iPhone but often a discrete notification of incoming calls or scheduled events require a vibrating alert. Unfortunately, the vibrating alert has been a very power hungry function relying on a micro-miniature DC motor spinning an eccentric weight at high speeds.

The launch of Melexis MLX90283 brings new capabilities in packaging and power efficiency to this ubiquitous feature. This single phase brushless vibration motor driver IC will allow increased battery life while still delivering strong and clearly detected vibrations. It can also benefit applications for force feedback or haptic interfaces in gaming systems, both handheld and home systems.

High motor efficiency
The MLX90283 integrates Hall-effect sensor, digital 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 start-up reliability. The operating voltage range from 1.8V to 3.6V perfectly suits 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 switch-over using a 'Freewheel delay' of 92ms (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 at the core of the high efficiency levels achieved with the MLX90283.

Start-up reliability
Another chronic issue in single-phase brushless motors is related to incorrect starting position, referred to as the 'dead point' 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 'dead point'. Thinner and smaller motor designs like those needed for vibration motors inevitably reduce the cogging plate efficiency. This can lead to unreliable start-up 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 in 'Active Start' 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 optimized for one rotation direction. Reverse rotation decreases 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 7/8 duty cycle, hence favors initial rotation direction and keeps the designed motor performance. The device exits the 'Active Start' mode once the rotation is detected by the Hall sensor in the IC.

Melexis 'Active Start' is an innovative technology exclusively developed and implemented at Melexis that solves the 'dead point' issue while favoring initial motor rotation direction.

The MLX90283 device is provided in a Chip Scale Package, the smallest packaged IC configuration available, said Melexis. The CSP saves valuable PCB and housing space allowing smaller integration. The footprint of the new Ultra-Thin QFN package is only 3mm? (1.5mm 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 miniaturization and ultra-slim designs.

The MLX90283 (part number MLX90283ELD) is priced at 34 cents in 5,000 quantities. Engineering samples are available upon request.

- Henri Arnold
EE Times Europe

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