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I am replacing the DRV8871 with the DRV8256P to drive a DC motor. The DRV8871 uses a slow decay current regulation method: I would like to continue using the slow decay method on the DRV88256P.
Per the data sheet:
7.3.3 Decay Modes
During PWM current chopping, the H-bridge is enabled to drive through the motor winding until the PWM current
chopping threshold is reached. This is shown in Figure 7-4, Item 1.
Once the chopping current threshold is reached, the H-bridge can operate in two different states, fast decay or
slow decay. In fast decay mode, once the PWM chopping current level has been reached, the H-bridge reverses
state to allow winding current to flow in a reverse direction. The opposite FETs are turned on; as the winding
current approaches zero, the bridge is disabled to prevent any reverse current flow. Fast decay mode is shown
in Figure 7-4, item 2. In slow decay mode, winding current is re-circulated by enabling both of the low-side FETs
in the bridge. This is shown in Figure 7-4, Item 3.
The DRV88256P has a quad-level Decay pin. It has four options:
Table 7-5. Decay Mode Settings
DECAY DECAY MODE
0 Smart tune Dynamic Decay
1 Smart tune Ripple Control
Hi-Z Mixed decay: 30% fast
330k to GND Fast decay
How do I select Slow decay?
Thank you.
Hello Robert,
The best option will be to select the Mixed Decay 30% decay mode. In this mode, the device is in fast decay for 30% and the remaining in slow decay until during the regulation decay time. This is shown in the following diagrams below.
In this mode, the decay is not slow decay 100% of the time but 30% fast and 70% slow. The main benefit of this mixed decay is that the current decays faster than purely slow decay but do not have the high current ripple which is present when running at 100% fast decay.
Regards,
Pablo Armet
Dear Pablo,
Thank you for your help. I will design in the mixed decay mode and see how it performs in our application.
Sincerely,
Robert Lasnik