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DRV8701: V_offset

Jan Bock
Intellectual 2395 points
Part Number: DRV8701

Hello, 

my question is regarding the maximum value of V_off in the datasheet. I wonder why the max offset voltage is 5 times (so five sigma) higher than the typical value.  Under which conditions (e.g. aging, quality issues) does this high offset voltage occur. How do I have to consider this maximum value in my calculations and simulations, because I saw that the V_off  has an significant impact on the overcurrent protection. 

Best regards 

Jan Bock 

  • 0
    TI__Guru** 114905 points

    Hi Jan,

    The V_off has no impact on the internal overcurrent protection of the DRV8701. The values VDS(OCP) and VSP(OCP) are used to determine the internal overcurrent protection values.

    Are you planning to use the SO pin to measure the current for overcurrent protection? If yes, we recommend performing a two point calibration if possible.

  • 0 in reply to Rick Duncan
    TI__Intellectual 2395 points

    Hi Rick,

    the SO pin is not in use. The question is more related to the life cycle off the end-product, to guarantee a long term functionality and to define the worst case situation in the specs of the device.

    • When do I have to expect this maximum V_off will occur. Is the 250mV related to batch fluctuations, temperature effects, quality issues or aging effects?
    • In the application a 5mOhm shunt is used at 12A load current. Would it be better to take a smaller shunt with pre-amplification to decrease the influence of V_off or is there a motor driver with same specs but more accurate current sensing?

    Best regards,

    Jan 

  • 0 in reply to Jan Bock
    TI__Guru** 114905 points

    Hi Jan,

    Sorry to confuse you. The SO pin is part of the current regulation circuit that can be measured by an external ADC if desired, or used internally with VREF.

    The overcurrent protection circuitry is 4 separate VDS monitors (one for each FET) that disables the FETs if the VDS exceeds 1V typically.

    Just to confirm, are you connecting a 5mOhm shunt from SP to SN of the DRV8701 as shown in the functional block diagram?

    If yes, are you then connecting VREF to ~1.2V to limit the current by setting the Ichop current?

  • 0 in reply to Rick Duncan
    TI__Intellectual 2395 points

    Hi Rick, 

    yes the 0.005Ohms shunt is connected to the SP to SN Pin. The device is used in 12V and 24V applications. Typically the driven motor needs 12 A supply current. The Vref is connected to VADD via voltage divider. To define the worst case conditions in the datasheet of the device, I need more information about the chopping current which depends on V_off by following equations from DRVs datasheet (see an example below):

    Best Regards,

    Jan

     (13.5A;  14.5A) 

  • 0 in reply to Jan Bock
    TI__Guru** 114905 points

    Hi Jan,

    The current sense amplifier was intended to protect the motor against higher than expected currents, but not accurately regulate the current.

    The offset variation has two components; die to die variation and temperature variation. 

    Please note the offset is difficult to determine because the offset is measured at SO, which is the Voff * Av.

    If more accurate current regulation is required, there are a couple of steps to consider assuming the system has the ability to read the SO (ADC) and set VREF based on the reading (DAC).

    1)      The SO can be read at 0A, which will provide the offset voltage (V_off)

    2)      The VREF voltage can be set to the desired VREF voltage + V_off.