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Tool/software:
Hi all,
We are developing a PWM controller for 24V motors and I have some questions.
I appreciate your help in advance
Thank you very much!
Hi Ian,
Is R_EXT R33 or R32 in your schematic image?
The SN74 buffer has a 40mA source current with 1V across it, which is roughly 25 ohms of output resistance (figure 5 below). You can use this as a ROH_buf estimated value.
Your calculation of 112 is about right. The series combination of ROH_buf, R33 and R26 just need to limit the input current from 3.3V into the ~2.2V input diode to IF=~10mA. The Vf of the input diode depends on temperature, but there is a wide If tolerance to account for this variation. As long as the input current stays above 5mA and below 20mA, the device will work fine.
R32 is the gate resistor, and the lower value you use, the higher the switching speed and efficiency. Your might want to experiment with lower values if your switching frequency is high.
Best regards,
Sean