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Original question:

DRV411: DRV411 H-Bridge driver and compensation issue

DRV411: Functioning of H-Bridge Driver and Offsets for Small Current Sensing Applications: 20 mA - 10 A Range.

Balaji Subramanian
Prodigy 15 points
Part Number: DRV411

Dear Sir/ Madam,

We are building a project having the title:  " Closed - Loop Hall Effect Sensors for DC Residual Ground Fault Current Detection"

Exact Current to be sensed is :  20 mA  DC Current 

I have read the Datasheet and Application document for 50 and 100 A detection, but some concepts were not clear to me.

Following are the questions  :

1.]  a.] Does the H- Bridge Driver also function as an Amplifier ? Or the Amplification for Compensation is done by the "Pre - Driver" ?  Is the Gain Selection for Pre - Driver ?

           Can you please send a link or suggest a source where I can clearly understand the working of a H-Bridge Driver to drive Electromagnetic/ Compensation Coil ?

      b.]  How does the Closed Loop Compensation exactly work ? 

2.]  Will the Current Spinning eliminate the Hall Sensor Offsets for such Low Current Detection ? Apart from the Magnetic Core, which we have to design.

Below is an extra question just for understanding:

3.]  Will Earth's Magnetic Flux  need to be taken into account for such Low Current Sensing ? Will it create great offset in the Hall Sensor and finally in the DRV 411 or this will be mitigated through Current Spinning ?

Sorry if the questions are basic, this being my first time designing a Sensing Circuit.  Flux- Gate Sensors have better sensitivity but we are using Hall - Effect to obtain good shielding from noises or external fields.

Thank you very much.

  • 0
    TI__Mastermind 27690 points

    Hello Balaji,

    I am not sure if you will be able to measure the 20mA with this   You may be able to do it with calibration but the issues are going to be a few things.  Offset of your hall sensor, external magnetic fields and the gain of your magnetic core.  These will all play a role.  So the accuracy at those low currents may not be to good. 

    1.]  a.]The H-Bridge Driver drives the compensation coil which will always attempt to bring the hall effect sensor to a zero sensing field.  The gain option is there for compensation purposes.  Depending on the inductance of your compensation coil as stated in the datasheet.  You could simplify the compensation drive as a the pre and hbride as one gain stage.  Use the guidelines of the datasheet for driving different inductance as a starting point.

    b.]  Like stated above the device measures the hall effect sensor and makes a decision if the field is not measuring zero then it drive the compensation coil to make the measured field at the hall sensor zero.  Therefore after a sensor measurement is made it adjust the output of the compensation coil pins to drive it to zero.  This happens for low frequencies, as it takes time for the sensor to get measured.  If there is an offset in the hall sensor or the sensor measurement it will come into play at this point as it cannot distinguish from an error in field or error in measurement.  At higher frequencies the measurement cannot keep up but the core acts like a transformer and the current through the Rshunt is N (number of turns) smaller than the current being measured.

    2.] The current spinning will assist in lowering the offset of the hall sensor but not completely remove it.  This will be dependent on the sensor as not all hall sensors have zero offset or no drift.

    3.] You will need to account for any external magnetic field when measuring low currents.  This will also depend on your core design and placement of sensor.  The core will give you your magnetic gain which will tell you how much sensing current will change your field.  There are trade-offs as you can increase the inductance and increase the saturation possibility of the core as you decrease the air gap.  The air gap and core material will help determine how much your external fields are redirected from the sensing locations.  You need to calculate the core gain and offsets you will expect with the spinning to get the true error you will get from an external field source.