DRV8703D-Q1: Brake definition

Hey Samuel,

Thanks for posting in this forum.

The e2e post you referenced was for a BLDC motor. While the idea may be the same there are differences when it comes to BDC (brushed DC motors).

Speaking of BDC motors "passive brake" refers to braking behavior of the motor while its terminals are shorted together. Under this condition when motor tries to spin by an external torque, the motor generated back EMF, BEMF. Because the motor is shorted the BEMF results with opposing current flow to counteract the external torque. Such braking torque would be a function of the motor characteristics its coil design and permanent magnet strength, This does provide resistance to an outside torque but it has limitation. For example it may not be able to stop a slow creep unlike a rigid electromechanical brake. 

Now, looking at the DRV8703-Q1 Full-Bridge driver (note this does not apply to the DRV8703D-Q1 because this is an Half-Bridge driver), as indicated by "2" current flow while both low-side FETs are conducting it effectively behaves like shorting the motor coil, meaning the BEMF current would flow via the two low-side FETs causing the motor to brake. This applies to when the motor is not actively driven.

During a PWM drive during the PWM on of the duty cycle the current "1" increases via the motor coil according to its supply voltage, R and L. During the off duration of the PWM both low-side FETs are switched on to allow the recirculation current to decay, aka slow decay. See below image for visualization.

Samuel Heckenthaler said:

Or the power/efficiency of the Brake low-side slow decay can't be compared to a disc brake on a bike. 

While a passive brake provides resistance to the spinning motor and provides some level of holding torque while the motor is not driven it does not behave like a mechanical disc brake on a bike. For this purpose you may have to use a purpose built electromagnetic brake. Thank you.

Regards, Murugavel