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Clarification on TVOUT for DM365

Harini Ramanujam
Prodigy 635 points

 

Hi,

Can you help clarify on the TVOUT (pin A10) for DM365. According to the datasheet, the TVOUT pin can directly drive composite video input of TV with 75 ohm input impedance, the TI demo board also has a passive filter on the TVOUT. What will happen if the TVOUT gets shorted to ground by any chance? Does the TVOUT internal buffer have current limiting circuit built in? Will the external short damage the part? Thanks.

 

  • 0
    TI__Genius 9490 points

    1) Yes, the TVOUT pin can directly drive the composite video (it already includes an internal buffer fyi:)

    2) The video buffer doesn't have protection circuit of short connection. If there is short circuit on the TVOUT, the buffer will have large current.

    So, the spec doesn't allow the short circuit.

     

    regards,

    miguel

     

  • 0 in reply to Miguel A. Alanis
    TI__Prodigy 635 points

    Thanks Miguel. Do we have any recommendations for a workaround for customers that need the shortage protection. Will this method work - limiting the current of TVOUT while not sacrificing output amplitude by adding a resistor on TVOUT pin before VFB input. See the drawing below:

     

      

  • 0 in reply to Harini Ramanujam
    TI__Genius 9490 points

    The drawing did not come through..

  • 0 in reply to Miguel A. Alanis
    TI__Prodigy 635 points

     

    Miguel,

    Thanks for your help. Based on the recommendations, below is what was proposed.

     

    The suggested solution of adding the resistor will not work. The buffer is on 1.8v supply, in order to have 75 ohm series and still deliver full video level to 75 ohm termination, swing needs to double. I think this will require 2.5V swing, which is impossible with 1.8v supply. Below are some other options:

    1.You can put the series resistor after the feedback point, but that will reduce the swing on the output load.

    2. You can put a large ac-coupling cap (220uf). It is common in a lot of video drivers.

    3. Adding an external video buffer is another option.

    4.If those solutions do not work, we can recommend an external short circuit protection, but that will require some discrete components (2 transistors, a resistor) and some distortion.

     

  • 0 in reply to Harini Ramanujam
    Expert 1945 points

    Hi,

     

    Could you please provide the register settings to bypass the normal tvout and having composite output through configuring DAC?

    I configured vdac_config register as 0x81141e7 and dacsel register as 0x0.But it didnt bypass the normal tvout, as I was getting signal at tvout and nothing at the Y, Pb, Pr outputs.

    What are the hardware requirements for bypassing this tvout?

    Regards

    tvs

  • 0 in reply to Sudheesh
    TI__Genius 14405 points

    tvs and Harini, are you working on the same issue here?

    Not sure if i've understood the entire issue. What exactly does the customer want to do?

    I am assuming you have video buffer connected to the  Y,Pb,Pr outputs.

    Could you please check if you can get component signal out of these outputs?

     

  • 0 in reply to Paul.Yin
    Expert 1945 points

    Hi Paul,

    My requirement was bypassing normal composite output at the tvout pin of DM365, and getting it at Y pin. 

    I could achieve that through the following settings.

     

    //VMOD

    memtool 0x01c71e00=0x3 //or 0x403

    //CLKCTL

    memtool 0x01c71f40=0x1

    //VDAC_CONFIG 

    memtool 0x01C4002C=0x081141e7

    //DACSEL

    memtool 0x01c71ef4=0x210

     

    Regards

    tvs