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PWM Buffer Component Selection
Requirement
4-channel
Vin >= 6V
Can buff 3.3V PWM https://docs.holybro.com/autopilot/pixhawk-baseboards/pixhawk-baseboard-ports
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Servo Motor Market Research
Most servos can handle 3.3V & 5V logic levels but some high torque servos only take 5V logic level and doesn’t respond to 3.3V PWM signals. Therefore we want to buff the 3.3V PWM signal coming from the flight controller (Pixhawk 6) to 5V.
Google search “step down servo motor PWM signal from 5V to 3.3V” gives guides on the opposite… so we don’t need to worry about 5V PWM not compatible. Plus we have jumpers to get around the buffer if we really want a 3.3V PWM signal.
Requirement
4-channel
5V Vcc
Level shift PWM signal voltage from 3.3V to 5V https://docs.holybro.com/autopilot/pixhawk-baseboards/pixhawk-baseboard-ports
Selected SN74AHCT125PWR for cheap price and small package.
5V LDO
Current consumption of the buffer is 25mA max, Vdrop = 6V- 5V = 1V, Ploss = 25mW worst case. No need for a big package to dissipate heat. Selected TLV76050DBZR for cheap price and small package.
Input Pulldown Resistors
I couldn’t find our flight controller ( Pixhawk 6) PWM rail leakage current. Assuming 100uA leakage current from Pixhawk, 4.7k pulldown will create a 0.47V input, which is below the 0.8V logic low threshold.
Selected 4.7k as pulldown resistor value.
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