Prevention of concurrent PWM or PSM commutations of power signals driving a voice coil motor (VCM) to reduce S/N ratio degradation of low level signal lines of a R/W head

A method of reducing concurrent or quasi-concurrent commutations of a pair of phase shift modulation (PSM) drive signals of an output bridge stage driving a voice coil motor comprising:

• establishing a threshold level of a programmed current level for the voice coil motor;
• and
• if the programmed current level is lower than the threshold level, enhancing a time offset between commutation edges of the pair of PSM drive signals by a desired time by at least
• broadening a width of pulses of one of the pair of PSM drive signals, and
• narrowing a width of pulses of the other PSM drive signal while maintaining an individual duty-cycle of each PSM drive signal constant;
• the programmed current level for the voice coil motor has one of a positive and a negative sign, and the broadening and narrowing of the pulses of one and of the other PSM drive signal, respectively, is of a greater amount than the desired time when delivering an output current of the negative sign.

A circuit for producing a pair of PSM drive signals for an output bridge stage driving a voice coil motor coupled thereto, the circuit comprising:

• a digital generator outputting a pair of triangular reference signals having opposite phase, the pair of triangular reference signals being synchronous with a system clock signal applied to an input of said digital generator;
• a digital comparator receiving at respective digital inputs the pair of triangular reference signals and a periodic modified digital input signal, said digital comparator outputting the pair of PSM drive signals resulting from a comparison of the periodic modified digital input signal with the pair of triangular reference signals;
• an input digital value modulator receiving
• at a first input, a programmed input digital value,
• at a second input, one of the pair of triangular reference signals output by said digital generator, and
• at a third input, a programmed edge shift value for outputting the periodic modified digital input signal modulated by comparing the programmed input digital value, modified by the programmed edge shift value, with the input triangular reference signal,
• the periodic modified digital input signal being input to a digital input of said digital comparator in lieu of the programmed input digital value for producing the pair of PSM drive signals.

A voice coil motor driving circuit comprising:

• an output bridge stage;
• a control circuit coupled to said output bridge stage for reducing concurrent or quasi-concurrent commutations of a pair of phase shift modulation (PSM) drive signals of said output bridge stage driving a voice coil motor by at least
• establishing a threshold level of a programmed current level for the voice coil motor;
• and
• if the programmed current level is lower than the threshold level, enhancing a time offset between commutation edges of the pair of PSM drive signals by a desired time.