Tool/software:
Hi everyone,
We are developing a PWM controller for 24V motors and I have some questions
1. To calculate R_EXT I don't quite understand the ROH_buf parameter.
2. I need to know if I am choosing the right Rg for the MOSFET or are there other considerations to take into account?
I appreciate your help in advance
Thank you very much!
Hi Ian,
Does R_EXT represent the sum of R33+R26 in your schematic? I agree with your math, although Vf will vary with temperature. A 10mA typical ensures that the input current will stay within 5mA and 15mA across operating temperatures.
Based on the SN74 datasheet, I think the Roh looks closer to 25 ohms typical, but it should not matter too much.
For R32, lower Rg will increase the switching speed and efficiency. This is great if you are targeting a >30kHz switching frequency. However, the faster dV/dt might expose some resonant frequencies in your high voltage switch node, leading to overshoot and ringing. If you were using a supply voltage close to the MOSFET Vds rating, you would have to be careful when reducing R32 to avoid transient overvoltage that will cause the FETs to enter destructive avalanche breakdown.
Best regards,
Sean
