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Original question:

BOOSTXL-DRV8305EVM: Default (power-on) current shunt amplifier gain seems to be 20V/V rather than 10V/V as expected

BOOSTXL-DRV8305EVM: Default (power-on) current shunt amplifier gain seems to be 20V/V rather than 10V/V as expected (continued)

Eric Wu
Prodigy 40 points
Part Number: BOOSTXL-DRV8305EVM
Other Parts Discussed in Thread: LAUNCHXL-F28379D, , DRV8305, SYSCONFIG

Following up on the previous thread, I am still seeing an apparent 20V/V gain on the DRV8305EVM rather than 10V/V.

I have routed a DC current through Phase A current sense resistor R4=0.007ohm, using my switchmode power supply at constant current mode.

First image is with zero current with CH1:ISEN_A CH2:N_1

The cursor was used to read the current sense resistor voltage, which was zero. The current sense amplifier output was 1.67V.

Next Image is with 3.998A current with CH1:ISEN_A CH2:N_1

Cursors were used to read the current sense resistor voltage, which was averaging -28.2mV as expected. The current sense amplifier output voltage was 2.24V.

Considering the offset, the gain from the current sense amplifier was (2.24V-1.67V)/(0.0282V-0V) = 20.2V/V

I took the same measurements using a Hioki DT4252 multimeter and obtained the following values:

(2.256V-1.695V)/(0.030V-0.001V) = 19.3V/V

In both cases the default gain on the amplifier appears to be 20V/V, corroborating the results from the last thread. Again I have not made any SPI commands, and I have only turned on the GATE_EN input to turn on the amplifier.I am providing 12V to PVDD and Vreg&Vref is 3.381V according to my multimeter.

I have observed the issue on two boards already. Has anyone else observed this issue or can reproduce it?

Thanks.

  • 0
    TI__Expert 8460 points

    Hi Eric,

    Thank you for posting to the Motor Driver's forum!

    I will need some time to look into this and will aim to provide a response by the end of next week.

    Best,

    ~Alicia

  • 0
    TI__Expert 8460 points

    Hi Eric,

    I know you mentioned that you are not doing any SPI commands, but as a sanity check, could you please do a SPI read of the Shunt Amplifier Control register (0xA) just to verify that the GAIN_CSx bits are set to the default value b'00 and not b'01.

    Best,

    ~Alicia

  • 0 in reply to Alicia Rosenberger
    Prodigy 40 points

    Hi Alicia, I read the register and confirmed that the values for all 3 registers are b'00.

  • 0 in reply to Eric Wu
    TI__Expert 8460 points

    Hi Eric,

    Can you short SPx and SNx together, force a voltage on the shorted inputs and read back the voltage output of the corresponding SOx?

    Best,

    ~Alicia

  • 0 in reply to Alicia Rosenberger
    Prodigy 40 points

    Hi Alicia,

    If I short the sense resistor it will short any voltage source I put on it and have a zero voltage, won't it?

    I presume you mean to open-circuit/remove the resistor and apply a direct voltage to the shunt amplifier input?

    Thanks,

    Eric 

  • 0 in reply to Alicia Rosenberger
    Prodigy 40 points

    Hi Alicia

    I removed sense resistor R6 and soldered jumper wires to SP3 and SN3. Using a voltage divider to easily control the voltage from my switching power supply, I forced the voltage across SP3 and SN3.

    Without no launchpad connected (EN_GATE jumpered to 3.3V) I got the following readings on the current sense circuits:

    V_sp3sn3=0V V_isenc=1.694V

    V_sp3sn3=0.053V V_isenc = 1.169V

    (1.694-1.169) / (0.053 - 0) =  9.91 V/V gain

    With LAUNCHXL-F28379D connected (EN_GATE jumpered to 3.3V) I got the following readings on the current sense circuits:

    V_sp3sn3=0V V_isenc=1.685V

    V_sp3sn3=0.052V V_isenc=0.66V

    (1.685-0.66) / (0.052 - 0) = 19.71 V/V gain

    Somehow connecting the BOOSTXL-DRV8305EVM to the LAUNCHXL-F28379D causes the gain to be doubled. I am not sure why this would occur.

    A couple notes about the setup: PVDD = 12.0V

    on the Launchpad, JP1, JP2, JP3 are all removed, JP4, JP5, JP6 are all connected.

    Thank you.

  • 0 in reply to Eric Wu
    TI__Expert 8460 points

    Hi Eric,

    I will need to look into why connecting the EVM to the launchpad would cause the gain to double as you are seeing. I will aim to provide an update on what I have learned early next week.

    Best,

    ~Alicia

  • 0 in reply to Alicia Rosenberger
    Prodigy 40 points

    Hi Alicia,

    I just did another deep-dive into troubleshooting. It turns out that SPI is being written on powerup by the F28379D Launchpad, which I was able to confirm with oscilloscope readings on the SPI pins. I did not see any register changes before because I was checking the Boosterpack alone using another protocol tester. So it is unrelated to the DRV8305 EVM itself.

    A colleague was using the same boards and I suspect they programmed the SPI write code into the flash. Because I am debugging from RAM in CPU1, the SPI write code may still be in CPU2 flash and is being executed anyway, even although SPI is not enabled in Sysconfig. Is there a way to prevent CPU2 from executing during debug, short of erasing the flash memory, or preventing this sort of side-effect?

    I will try to confirm by swapping out with a brand new Launchpad.

    Thanks.

    Eric

  • 0 in reply to Eric Wu
    TI__Expert 8460 points

    Hi Eric,

    I will need to look into this and will get back to you the beginning of next week.

    Best,

    ~Alicia

  • 0 in reply to Eric Wu
    TI__Expert 8460 points

    Hi Eric,

    I would recommend posting your question about debugging with CPU1 vs CPU2 to the C2000 forum as they will be more knowledgeable on this topic.

    Best,

    ~Alicia