1.  Field of the Invention
The present invention refers to a device for driving electric motors, particularly for brushless three-phase motors. 
2.  Description of Related Art
Today's development in the field of computers and telecommunications is oriented more and more towards the production of smaller and lighter electronic equipment which is capable however of memorizing a remarkable quantity of data, such as palm computers or mobile phones, etc.  This equipment has two fundamental characteristics in common: power supplying with lighter and lighter batteries and the need for large memory banks to store the data. 
Today the cheapest devices for memorizing enormous quantities of data are the hard disks (HD) whose dimensions are becoming smaller and smaller.  The HDs and in any case magnetic memorization systems require mobile mechanical parts driven by motors that do not always meet the need for low energy consumption, an indispensable characteristic for battery powering.  In fact low energy consumption is essential in order to have batteries whose dimensions are smaller and smaller and which weigh less and are suitable for portable electronic equipment. 
As energy is linked to the supply voltage of the equipment, lowering the consumption of energy equals lowering the supply voltage of said equipment; for example today we have passed from supply voltages at 5V to voltages of 1.5V. With such reduced power voltages, the driving of the HD motors becomes difficult to actuate because of the voltage that is generated on the motor windings for the counter electromotive force (VBEMF) induced by the rotation itself inside the magnetic field produced by the polar expansions of the permanent magnets present in the motor.  This counter electromotive force depends on the rotation speed of the motor itself according to a linear relation: VBEMF=Ke*W where W is the rotation speed of the motor in rad/s and ke is a constant depending on the characteristics of the magnetic field and of the windings and is measured in V/(rad/s).  Therefore a low VBEMF corresponds to low rotation speeds, while a high VBEMF corresponds to high rotation speeds. 
Given that the motor is generally driven by a power stage, the variation of supply voltage of the motor is carried out by adequately varying the duty-cycle of the power stage powered by a voltage power which is in any case higher than the maximum supply voltage supplied by the power stage to the motor. 
If, however, in some applications where the voltage supplied by a battery to the driving stage of a motor is low, the counter electromotive force can become comparable to or even greater than the supply voltage of the motor, and therefore it can become impossible to forcibly introduce current into the motor and therefore transfer the energy needed to it to reach and constantly maintain the rotation speed requested by the application.