Originally Posted by
MechanoMan
Also note that whether or not Sync Rect is used, the fast decay current path involves making current go the opposite way on a shunt which puts the voltage to be sensed OUTSIDE the rails. Most hardware has a problem with this! Without specific hardware capabilities to deal with this, at best the signal will be unreadable while outside the rails, second worst it will mess with readings even after the sense voltage comes back within the rails (op amps need a period to recover from beyond-the-rails excursions for example), and at worst it can damage the chip.
It seems that when you're doing fast-decay from a higher current level to a lower, but the direction of the coil current is the same, if you don't have the ability to measure current then the controller is a bit blind and could overshoot or undershoot because it doesn't know when to switch back.
In fact, note that in recirculation mode (both lowsides on) the current sense resistor will read nothing since the coil current does not pass through ground at all. Yet the controller ideally needs to know what the current has decayed to. It shouldn't be allowed to decay too far. If we switch on the high side to check, there's a blanking interval which means the high side will be on for a certain fixed period, increasing current, before we can check it. This can raise the current beyond the target point and depending on the system this could happen repeatedly, raising the current each time.
Actually if you put a low side current sense resistor on EACH half-bridge, you can still read current in recirculation mode. One sense voltage will be beyond-the-rails though, the other will have the same magnitude but opposite polarity making it more readable.