generating 0.3V, 4Mz signal from microcontroller (ntsc / pal synch)

[Jaunedeau]

Hi,

  I want to output a csync signal to a SCART connector (75R terminated, csync only since color will be on RGB pins) from an stm32h450.

  I am very scare on space on the board and really would avoir using an opamp or active buffer.

  An obvious solution would be this :



  The line is terminated on each end with 75R, resulting in 37.5R, and with 3.3V output on the GPIO, the TV should see 3,3×37,5÷(350+37,5) = 0.31...V max, close enough. The MCU would see 385.5R and output about 8.5mA, that's ok.

  But I've been told it could be a good idea to add a diode like this :


 
  In this case, value of R6 is less critical : if the voltage goes above 0.3V, the diode would draw current, rise the voltage on R6 which will have a higher voltage drop, and the output voltage will be less sensitive to heat, psu, ...

  But looking at some diodes datasheet, if I take e.g. an SS26 diode, it seem hard to predict how much current it will draw at about 0.3V, something between 10 and 90mA... I do not understand how I am supposed to choose the resistor value.

  Should I use a diode for such a low current / tension or does this solution not apply here ? If I should, how should I choose the diode and resistor ?

[wasedadoc]

The obvious solution is fine. No need for any diode.  The 0.3 volt level is not at all critical. 

[eleguy]

The output impedance of this divider would be ~62R which is out of spec (that is +-10%?). But maybe it is close enough. Also depends the length of the cable. Another thing to be considered is the rise time of the signal but that should not be problem in these speeds. One can always put a little cap in parallel of R6 if tuning is needed. I have not worked in the field of SCART signals. These are just thoughts that came to my mind.