Other Parts Discussed in Thread: LAUNCHXL-F28379D, DRV8305
I am operating the BOOSTXL-DRV305EVM with a LAUNCHXL-F28379D to operate a 3-phase inverter with default settings in the DRV8305 (power up with no SPI write commands), and I understand the gain of the current shunt amplifier is 10V/V. With the 7mOhm shunt resistors the total current gain should ten be 0.07V/A at the output of the shunt amplifiers. The inverter is driving a 500uH 3-phase load.
I noticed that I am reading currents from the shunts that are double what I expect. I checked the shunt voltage and indeed, it seems that the voltage is being amplified by 20V/V by the amplifiers. Here is the output for an approximate 4.0 A peak output current at 60Hz:
CH1(10mV/div) is the voltage measured across the shunt resistor when the PWM operates at 12.5kHz. The peak to peak value is approximately 29.6mV.
CH2(500mV/div) is the voltage output of the shunt amplifier SO1/ISEN_A on the Boosterpack. The peak to peak value is 539mV.
The gain is closer to 18V/V (20)?
To eliminate the effect of PWM noise I also measured the actual phase currents using a current probe, and compared to the value read by the ADC, sampled when the lowside phase is turned on. The raw 12-bit value is output back on the DAC pin, both with the same 3.0V reference, so the DAC represents also the raw voltage read by the ADC.
CH1, with 10mV/A, shows approximately 2A peak current.
CH2, with 595mV peak to peak, and considering the combined gain of the amplifier(10V/V) and shunt resistor (0.007Ohm) of 0.07A/V, yields 0.595/2/0.07=4.25A peak. Once again this confirms that the gain actually seems to be 20V/V.
I have also done the same analysis at 4A peak current and obtained the same result.
Is it possible that the default shunt gain is actually 20V/V rather than the 10V/V listed both in the datasheet and the EVM manual?