Drivers Fail for Understandable Reasons

When an integrated motor driver seems to misbehave, the cause is usually not a random failure but one of a few systematic problems: a motor that will not turn from a disabled driver or a low current limit, a driver that runs hot from a poor thermal path, a current limit that does not work from a wrong sense resistor or reference, or a protection trip from a short or a stall. This article presents a method for diagnosing the common issues in Allegro stepper and DC motor drivers, so an engineer can move from symptom to root cause quickly.

The Motor Will Not Turn

A motor that stays still is often held off by a control condition rather than a fault. Check the supply voltage against the driver range, the logic supply and the enable and reset pins, the current-limit setting and the step and direction signals. Confirm that the driver is enabled, that the logic level on the step pin reaches the threshold and that the current limit is not set so low that the motor cannot develop torque. Measure the supply and the logic with a scope during start-up to see which condition is missing.

Stepper Phase Sequence

A stepper that buzzes or vibrates without turning usually has a wrong phase order or a step rate that is too high for the load. Confirm the phase connections against the datasheet and start at a low step rate, then increase it. A wrong microstep setting also reduces the torque, so confirm the microstep pins match the intended resolution.

The Driver Runs Hot

The first check for unexpected heat is the output current and the duty. The dissipation in the driver scales with the current squared and the on-resistance, so measure the current and compare it with the rating, and confirm the current limit is set correctly. If the current is as expected but the driver is still hot, the fault is in the thermal path. The QFN and SOIC packages use an exposed thermal pad, so confirm the pad is soldered to the ground plane with enough copper area and vias, and check the ambient and the airflow. A poor solder joint on the pad is a common and invisible cause of overheating.

Thermal Duty Cycle

A driver that is fine in a bench test can still overheat in a sealed enclosure with a duty cycle, so measure the case temperature at the worst case and compare it with the thermal limit. Provide airflow or a larger copper area where the duty is high.

The Current Limit Does Not Work

The current limit is set by the sense resistor and the reference voltage, so a wrong sense resistor value or a mis-set reference gives the wrong limit. Confirm both against the datasheet, and remember that the decay mode affects the ripple and the average current. A fixed off-time regulator, as in the A4988, and an adjustable PWM limit, as in the A4950, work differently, so read the relevant section for the part. Measure the coil current with a current probe to see the actual waveform.

Protection Trips

A driver that trips its protection when the load seems fine is usually seeing a fault it was designed to catch. Check for a motor short to ground or to the supply, an over-current from a stall or an inrush, and an over-temperature. The driver flags over-current, short-to-ground, short-to-supply, under-voltage and thermal faults, so trace which one is active and fix the cause rather than disabling the protection. A motor lead short is the most common fault, often from a pinched wire or a damaged winding.

A Systematic Method

Work from the simple to the complex: confirm the supply, the enable and the current limit, then the phase order and the step rate, then the thermal path, then the protection state and the motor itself. Keep a known-good driver and motor as a reference, measure the supply, the current and the case temperature of each new design on the bench and record them so a later change is immediately visible. BeiLuo supplies genuine Allegro drivers with an import declaration, a certificate of origin and a RoHS compliance file, and our FAE team can help you diagnose a fault and choose a design change that resolves the root cause.