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DRV8812: Designing with motor driver, Decay mode

Part Number: DRV8812
Other Parts Discussed in Thread: DRV8426, DRV8436, DRV8424

Hi Team,
I am using DRV8812 in one of our products.
3.3V will be out I/O logic.
The VM will be 40V generated from a boost converter. The boost converter will be stepping the voltage from 3.3V to 40V.
We are planning to drive a piezo blower with this driver.
1). We need to read the current consumed by the blower while working, so can I take analog readings from the sense resistors at the ISENSx pins to my microcontroller?.
2). What is Decay?. If my understanding is correct, we need to bother about decay only when the direction of the motor is reversed or the motor is stopedd. and decay is the method to making s path to release the stored energy in the inductive load. 
3). Which decay mode is preferred in my application?. 
4). If my understanding is correct, by the sense resister, we are setting the full-scale current (current chopping) and by xL0 and xL1 pins we are adjusting the % of the full-scale current to be flowing through the load(PWM chopping).
5). What is PWM chopping and current chopping?.


  • Hello, Shibin:

    1. Yes, you take analog readings from the sense resistors at the ISENSx pins to the microcontroller. Please add a filter to filter out the switching noise.

    2. When the output current reaches the regulation point, the motor driver has to disconnect the output load from the input source. But, the inductive load keeps going the same direction. The decay mode selects the inductive load re-circulation current path and decides the output current drop rate. It could be "making s path" or "o path" (turn-on both low side FETs or both high side FETs).

    For brushed DC motor in general, the decay mode just simply maintains the output current by turn-on both low side FETs or both high side FETs to reduce the voltage on the motor coil terminal. 

    3. If it is a new design, you can consider DRV8426 which supports the smart tune dynamic decay to automatically adjust the decay mode. If it is existing produce, you can check the output current waveform to see if it is what you want.

    4. Yes. xI0 and xI1 pins can adjust the % of the full-scale current to be flowing through the load(PWM chopping)

    5. PWM chopping and current chopping are same for DRV8812 because DRV8812 only does the current regulation. 

    Regards,

    Wang Li

  • Hi Wang5577,
    Thank you for your reply.

    The motor driver needs to disconnect the load from the input source only when the phase is reversed or anything like a given MCU right?.
    Not when the current is reaches the limit.Right?.
    The suggested DRV8426 has only a load voltage of 33V, in my case it is 40V.
    I am using this driver to drive a piezo micro blower.

  • Wang will respond within 24 hours.

  • Shibin,

    Disconnect may not be an accurate word. It is not physical disconnection; just stop driving the current beyond the regulation point. When the output current reaches the limit, decay mode control the current path in the switch to make sure the current not over the regulation point.

    If the input need 40V, please check DRV8436.

    Regards,

    Wang Li

  • Hi Wang,
    Thank you for your reply.
    The selected DRV8812 has a recommended supply voltage of 45V.
    Is there any problem in moving forward with DRV8812?.

  • Shibin,

    DRV8812 should be OK. But, DRV8436 is a new released device with flexible decay mode selection; high micro-stepper resolution; pin to pin compatible to DRV8424 (higher current option).

    Regards,

    Wang