Square Wave 6 Step Commutation Resistor Network Values?

[ericwil76]

Hello folks new here save the introduction gonna ask a lame question.

I am designing a motor drive for a brushless motor and need to set the resistor network values for my 6-step commutation sensing. I understand how it works but not the formula to determine the resistor values to supply voltage to each pin on the MCU.

The Vin or BAT voltage in this case for nets Motor A, B and C is (9 - 50.4V). This is 3-12S LiPo absolute low to high voltage cutoff ranges.

The MCU can only input up to 3.3V's on Pins P0.0, P0.1, P0.2 and P0.3.

I need safe resistor values so I do not over or underdrive the MCU pins. The Pin input voltage needs to be between +0.0V to 3.3V.

I know how to calculate the forwarded voltage across a resistor but I am not clear on the formula for the network.

Here is a picture of the network, the values listed are arbitrary and come from a reference design with different voltages.

Any help is greatly appreciated.

[trilerian]

3s-12s LiPo for a motor controller is a big range...  From looking at what you are doing, I assume you are building a sensorless controller and you are going to use zero cross detection for rotor position.  Ultimately you need to drop your max of 50.4V down to a voltage that can be seen by the mcu inputs.  In reality the voltage will not quite be 50.4V, but it will be close.  Looking at your diagram, look at R37 and R44, these two resistors make a voltage divider on phase A.  R37 is R1, and R44 is R2.  I would say something like 120kΩ for R1 and 7k5Ω for R2.  You will need the same values for each of the 3 phases of the motor.  Then the last bit on the left of your diagram are the resistors that tie all the phases together to create the virtual common.  These just need to be high enough to limit current on them. I've used 33k-100kΩ.  Then you will be comparing the phase wires to the virtual common to determine the zero cross.  You may want to look into low pass filters for the signals as well, they are extremely noisy.  Mind your low pass values though, it could change your perceived zero cross time.