Sign-Magnitude PWM
A drive method in which direction is given on a separate digital line and speed on a separate PWM signal. A duty ratio of 0 percent corresponds to a genuine stop and no current circulates in the winding when the motor is stopped; for this reason it produces noticeably less loss at low speeds than locked antiphase.
In this method one leg of the H-bridge is held in a fixed state according to the direction line while the other leg is switched with PWM. The duty ratio directly sets the magnitude of the average voltage applied to the motor, and the direction line sets its sign. The control logic is intuitive: one bit for direction, one number for speed.
The efficiency advantage comes from this: when the speed is zero the PWM is zero as well, no current circulates in the motor and no idle loss occurs. In locked antiphase, on the other hand, zero speed means 50 percent duty and there is a constant ripple current in the winding.
On the other hand a small dead band can be seen around zero: at very low duty ratios the motor cannot overcome friction and does not turn. This band is typically between 5 and 15 percent and is managed by defining a minimum duty in software.
On/off and direction-oriented applications suit this method very well. The logic of a module focused on direction and start–stop, such as the ST42, is essentially this: direction is handled as one command and motion as a separate trigger.
The context in which this term is used: DC Motor Driver Selection Guide