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DRV8706-Q1: Current Shunt Operation

Warren Muir
Intellectual 525 points
Part Number: DRV8706-Q1
Other Parts Discussed in Thread: MSP430F5529,

Dear Support Team

I am using a DRV8706 to drive a brushed DC motor controlled by an MSP430F5529. I have a 2mR shunt  between SP and SN and am monitoring the  SO pin with an ADC input. I have tried various configurations of the CSA register, but the amplitude on SO remains the same no matter what the load current is through the shunt. The noise obviously increases if I adjust the gain from 10V/V to 80V/V. Please see a scope trace of the SO output - I am pulsing the bridge at 7.782kHz.

From the datasheet, I understood that the SO output would be proportional to the current measured between SP & SN.

  • 0
    TI__Mastermind 44825 points

    Hi,

    Thank you for interested in DRV8706-Q1. It is strange no matter what of gain settings(10V/V to 80V/V), you did not see the change of So output.

    Here is my guess and check items.

    -AREF pin requires external supply (e.g 3V). It is supply for Shunt amp block.

    -IC_CTRL (addr 0x04) LOCK bit. If 110b, registers are locked.

    -Better to "CLR_FLT" is DRV has some fault condition.

    If you provide schematic, let me review it.

    regards

    Shinya Morita

  • 0 in reply to Shinya Morita
    Intellectual 525 points

    Thanks very much for responding Shinya

    Please find schematic herewith.

    1. AREF is attached to 3V3 (VDD_M1) as per the reference schematic for the EVM.

    2. IC_CTRL is not locked

    3. I clear CLR_FLT before writing to the CSA register

    DRV8706-sch.pdf

  • 0 in reply to Warren Muir
    TI__Mastermind 44825 points

    Hi Warren,

    Thank you for your reply. It is wired why your SO output does not change by motor current or amp gain settings. I am comparing TI EVM schematic and your schematic. 

    Not sure it is related to this issue however here is my findings.

    Here could be factor to reducing shunt amp signal amplitude.

    -R112/R110 is reducing signal amplitude of SO. 

    -R120 is 2mohm which is smaller than 7mohm on EVM

    -Set blanking time 0% if set others now.

    Otheres

    -Remove C70 1nF as trial.

    drv8706-q1.pdf

    regards

    Shinya Morita

  • 0 in reply to Shinya Morita
    Intellectual 525 points

    Thanks for your response Shinya

    WRT your responses:-

    1. The R112/R110 divider does drop the voltage, but this does not explain why the voltage doesn't change amplitude with different load currents. The output remains the same amplitude whether I am driving an average current of 1A or 5A. I will try remove R110 to see if this helps, but I don't believe it will.

    2. R120 I will try a larger resistor, but this does not make sense either. The voltage between SP & SN increases when I increase the load current from 1A to 5A, but the output at So remains the same.

    3. I have tried various values for CSA_BLK, but this does not make any difference to SO. I have left the blanking at 000

    4. I have removed C70, and this has no affect.

    5. I have not removed the snubber components as per the EVM schematic yet, but I don't think these would have an effect. I could try this if you believe it will help.

  • 0 in reply to Warren Muir
    TI__Mastermind 44825 points

    Hi Warren,

    Thank you for your reply. Actually I do not have clear idea why your SO does not change by motor current or gain settings. Your schematic looks very similar to EVM. Let me post more idea once I figure out.

    regards

    Shinya Morita