TMDS351

Riccardo Bortolato

Other Parts Discussed in Thread: TMDS351

Hello,

I designed a custom board and I am finding out that if I connect a sink (in this case an HDMI monitor) before powering the board on I get around 2V on the VCC and VDD rails.

Since I share the 3.3V rail with other stuff (e.g. microcontroller) I need to keep the rail clean until proper power is given through a supply.

What are my options? My only idea is to use a static switch to turn on the 3.3V on the TMDS351 programmatically.

On the other hand, why is this happening? I think the TMDS signals from the monitor are messing this up (I disconnected the i2c bus, no CEC and no HPD_SINK). Is there something I didn't consider?

Ciao,

Riccardo

over 8 years ago

0 Adam Torma over 8 years ago

TI__Mastermind 45575 points

Riccardo,

Thank you for posting your inquiry. The TMDS351 device is an active device and is supported in the consumer and computing forum. I will move this post to that forum to help you with your request.

Thank you,

Adam

0 Riccardo Bortolato over 8 years ago in reply to Adam Torma

Prodigy 40 points

Thank you.

For now I solved my issue by separating completely the power rails with an additional LDO for the TMDS351 3.3V rail only.

My board input: 5V, 3.3V (to microcontroller and other stuff)

to TMDS351: 5V, diode -> ~5V and generated 3.3V through LDO

I can confirm the HDMI monitor, when the board is powered down, has pullups on the TMDS pairs which make the TMDS351 output ~2V on the VCC and VDD pins.

Weird.

0 Moises Garcia over 8 years ago in reply to Adam Torma

TI__Mastermind 23815 points

Hi Riccardo,

Would you mind doing a quick test for me?
With the board not powered and disconnected from source and sink side
Could you apply 3.3V to one of the TMDS sink signals, and measure if there is any voltage level in Vcc?

Regards

0 Moises Garcia over 8 years ago in reply to Riccardo Bortolato

TI__Mastermind 23815 points

Hi Riccardo,

Could you try the test I proposed?
With the board not powered and disconnected from source and sink side
Could you apply 3.3V to one of the TMDS sink signals, and measure if there is any voltage level in Vcc?

Regards

0 Riccardo Bortolato over 8 years ago in reply to Moises Garcia

Prodigy 40 points

Hello Moises,

I tried doing what you asked and found ~0.7V on VCC and ~1.6 on VDD.

Ciao,

Riccardo

0 Moises Garcia over 8 years ago in reply to Riccardo Bortolato

TI__Mastermind 23815 points

Hi Riccardo,

This should be due to ESD diodes in TMDS outputs, connecting(powering up) a sink device before powering up TMDS351 is not a common scenario.

Regards

0 Riccardo Bortolato over 8 years ago in reply to Moises Garcia

Prodigy 40 points

Hello Moises,

I figured, but hoped there would be some sort of protection on the supply rail, looks like it's not the case. We will just modify our design then.

Thanks,

Riccardo

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