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Current Sense Amplifier issue

Part Number: DRV8353RH-EVM

I am using the DRV8353RH evaluation board to drive (6 x PWM mode) a brushed motor, so only using phases A & B which is working as expected (phase C is floating). The problem I have is that the output of both A&B current sense amps is always full-scale i.e. 1.65VDC (which is VREF). I am driving the motor with 35V normally, but even if I reduce it down to 12V and vary the motor-load (changing the motor current by up to 300mA) there is absolutely no difference in the current sense output voltage. I was expecting to see a waveform that has an amplitude relative to the current sourced, but it is always continually VREF until I drop the enable line low, in which case it goes to 0V (which is expected).

I haven’t altered the default gain setting, so it is still configured by 47K to ground which is 10V/V and I haven’t changed the sense resistors which are 7mOhm. When the motor is running with no load, the current is only 120mA. By my calculations, the current sense amp output should only be in millivolts.  I am measuring with an oscilloscope from the ISENA & ISENB points after the low pass filter section. I gave not added any extra resistors to these lines, do they need additional loading perhaps?

Please help.

  • Hello PY1,

    The DRV835x CSA is bidirectional, meaning VREF/2 corresponds with 0A, a voltage below VREF/2 corresponds with positive current (i.e. current flowing downwards through the sense resistor), and a voltage above VREF/2 corresponds to a negative current (i,e, current flowing upwards through the sense resistor).

    120mA * 7mOhm * gain = 8.4mV and so the CSA output will be VREF/2 (3.3V/2) - 8mV = 1.642V.

    Thanks,

    Matt