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TPS65150 - adding VCOM part to design?

Paul Schoenke
Expert 1375 points
Other Parts Discussed in Thread: TPS65150, TPS65146, TPS65100, TPS65140

Hello:

I have been working on a design to drive a TFT display from a TI DSP processor.  I looking to use TI TPS65150 to provide most of the voltage rails needed by the TFT panel.  The schematic pages for the circuits are attached.

In reviewing the display documentation, it looks like my current implementation to generate the VCOM voltage is not sufficient, and I need to use a VCOM buffer amp instead.  Some of the TI LCD power management ICs (such as the TPS65146) have this functionality built in.  I could switch to one of these parts, but I want to find out if I could just add the buffer amp.

Can you make a recommendation for this?

Any feedback or input is welcomed and appreciated!

Thanks again!

Paul

display interface schematic.pdf

  • 0
    TI__Guru 59735 points

    Hello Paul,

    According the attached schematic, you are using TPS65140, not TPS65150. Is this correct? If this is the case, TPS65100 is very similar to TPS65140 but includes a VCOM buffer. Please check if this device fulfills your requirements.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Expert 1375 points
    Thanks Brigette! I'm sorry that my query was not more clear. I am having difficulty understanding whether I need the VCOM buffer since I'm not clear on what exactly the VCOM buffer is for so I can determine if I need it in my design.
    Paul
  • 0 in reply to Paul Schoenke
    TI__Guru 59735 points

    Hello Paul,

    If you want to know what VCOM is for, please have a look onto this page: http://www.circuitstoday.com/liquid-crystal-displays-lcd-working if you scroll down to the picture "How LCDs work" you can find the negative electrode (C) there. This electrode is supplied from VCOM. This electrode is in a lot displays always on the same potential, so you would think that a resistor divider and a capacitor should be able to deliver this stable voltage.

    If the current into this negative electrode would be measured, you would observe spikes, but only a very low average value. The amount of average current and the height and width of the spikes are dependent on the panel used. So there are panels which do not need a lot current there, but the more pixels a panel has, the more likely is a need for higher peak currents. These higher peak currents could cause a problem, if the resistor divider is high resistive and does not allow the capacitor that I mentioned above to be recharged fast enough.

    So to decide if a VCOM buffer is necessary in your application, measure the VCOM voltage while your panel is working. If you can observe a significant change in the voltage, try a bigger cap or a resistor divider with a lower resistance first to check if the voltage changes on VCOM causes a problem. (You could as well use a power supply for VCOM to check if the current need is too high for your application). If the problem is reduced or gone with these tests, most probable a VCOM buffer could remove the problem.

    If the problem is not changed by the above tests, I think the issue is not coming from VCOM, but from one of the other voltages. Then you would need to check if bigger output capacitors on the other outputs would reduce or remove the issue.

    Best regards,
    Brigitte