Allegro Microsystems, LLC

Jose L Tallarico

Alberto Bilotti

A full-bridge driver, and especially an entirely integrated bridge driver, for driving DC motors in either direction is capable of automatically braking the motor when the motor direction is to be reversed. The bridge circuit is of 'H' configuration and has four transistor switches connected in bridge configuration and has two bridge output terminals to which a motor may be connected. A differential window comparator has the input connected to the two bridge output terminals, producing a high level binary signal only when the voltage across the two output terminals is within a small voltage range centered at about zero volts. A pulse-width filter circuit having an input connected to the window comparator produces a high binary output signal only when the comparator-produced binary signal remains high for greater than a predetermined time. The effect of the pulse-width filter in combination with the window comparator is to ignore the first occurrence during braking of zero motor voltage, and to recognize the second occurrence of zero motor voltage which indicates the end of the braking period at which time the motor has stopped, after which the motor can be operated in the reverse direction without creating large transient transistor and motor currents during such a transition of motor running direction.

DC-motor bridge driver with end-of-braking detector

A full-wave bridge driver has four driver transistors. A motor is connected to the output terminals of the bridge. A fault detector latches off the four driver transistors when the bridge current jumps essentially instantaneously to exceed a predetermined safe-peak value due to a fault occurring in the bridge. A comparator has a predetermined current threshold, that is less than the predetermined safe-peak value of the fault detector, and is connected to a bridge-current sensing resistor for, when the bridge current exceeds the current threshold, producing a signal proportional to the difference between the bridge current and the current threshold. A pulse-width-modulator is connected to the comparator means output, and during periods such as at motor starting when the bridge current gradually exceeds the comparator threshold value, chops the signal at the bases of the four driver transistors, varying the chopped pulse widths to hold the motor current at essentially the threshold current value. For faults that occur across the motor or between the motor and either of the DC-supply terminals, the fault current paths in the bridge do not flow through the inductance of the motor and produce a sharp rising bridge fault current that is essentially transparent to the pulse width modulator which clamps high motor start currents at a level below that at which the fault detector trips the latch and shuts off the driver transistors.

Bridge motor driver with short-circuit protection and motor-current

A bipolar chopping driver circuit has a pair of NPN totem-pole connected driver transistors with a fly-back diode connected across the NPN source transistor and a biasing resistor connected between base and emitter of the source transistor. A fullwave bridge driver may be comprised of two such totem pole circuits with the inductive load connected between them at the junctions between NPN source and sink transistors. A clamp circuit is provided in both cases, for preventing the voltage swings at the bases of the source transistors from exceeding the DC supply voltage that is connected to the collectors of the source transistors, thereby permitting operation at high chopping frequencies without excessive delay in switching load current and excessive power losses.

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Buenos Aires, Argentina

Jose L Tallarico