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TDP158: Output issue

Part Number: TDP158

Dear colleague,

The customer is using TDP158 as an HDMI driver, according to the datasheet, HDMI signal enters TDP158 after AC coupling.

They test 2 PC as HDMI signal source, PC output is connected to TDP158, TDP158 output is connected to the screen.

After PC0 output source, the screen shows normally after passing the TDP158, there is no image on the display after PC1 output source through the TDP158.

The output resolution is 1080P.

The test found that when PC1 is connected to TDP158, PC1 TMDS is no output. When PC1 is connected to the screen, PC1 TMDS output is normal.

Then a customer pulls up PC1 TMDS to 3.3V by 50Ω, then PC1 is connected to TDP158, PC1 TMDS output is normal, the pull up is before AC cap.

So the customer questions are:

1. Why PC0 can be output to TDP158 normally without a pull-up, but PC1 needs a pull-up? There is no mention of this pull-up in HDMI related design.
2. Is the AC coupling capacitor needed for the TDP158 front end?

Best Regards,

Rock Su

  • Rock

    1. AC coupling is required for the TDP158 since the TDP158 is a DP++ to HDMI level shifter.

    2. PC0 is possibly a DP++ source which is a AC-coupled HDMI. PC1 is possibly a HDMI source which is a DC-coupled HDMI. The HDMI 1.4b and HDMI2.0 standards define a DC-coupled 3.3 V common mode interface, so the PC1 is expected to see 3.3V common mode voltage. Since the DP158 is AC-coupled, the common mode voltage is 0V, then you need external 50ohm to 3.3V to set the correct common mode voltage.

    Thanks

    David