Output impedance of the SN65LVDS387 LVDS line drivers

Hi Mark,

Looking back into a driver stage of an LVDS translator the impedance seen is Hi-Z. If you are looking for a nominal value instead of a "Hi-Z" I will have to speak to the design team to get an idea as this will vary slightly between driver stack design and technology (i.e. a device done in one process may have a different impedance for the same device in a different process).

Dear Michael:

Thanks for your reply.

Yes, a nominal value would be helpful.

Regards,

Mark Stern

Hello Mark,

I still try to figure out the value for you. Since the part is a little bit older, researching the answer is a little bit more difficult.

To understand you question a little bit better: you are interested in the impedance between the differential lines, Y and Z on the LVDS device, is that correct? The output impedance parallel to the 100 ohm LVDS termination resisor.

Regards,

Till

Dear Till:

Yes, that is correct.

Regards,

Mark

Hello Mark,

we've set up a test for you in the lab to determine the internal impedance. It is kind of hard to measure, since the internal impedance is small compared to the 100 ohm termination resistor. Therefore we cannot give you a precise number.
The internal impedance (at room temperature) is in the region of around 50 kohm. Please let me know, if you have any further questions.

Best regards,
Till Ostermann

Dear Till:

Thanks for your help.

Regards,

Mark

Dear Till:

Do any of the device pins, e.g., the power pins, conduct away more heat than the other pins?

Regards,

Mark

Hi Mark,

usually it is the PCB, that conducts away the heat from the chip. So if i understand your question right, you want to know if any specific pin or pins get especially hot and need a bigger PCB connection.

I don't have any thermal dissipation data except the datasheet data in chapter 8.4 Thermal information. Since there are several GND and VCC connections on this IC, you will be fine, if all of them have a connection to VCC and GND. Usually the GND and VCC pins "cool" the IC the most, since they most likely have the biggest copper connection.

Let me know if that answers your question.

Regards,

Till

Dear Till:

Thanks again.

The chip will be used in a vacuum environment, i.e., no convection cooling. Heat conduction will be primarily through a copper plane directly under the chip.

I am trying to get a feel for whether connecting the chip's six ground pins to that plane will increase heat conduction enough to justify the effort to electrically insulate the board mounting.

Regards,

Mark

Hello Mark,

That is a very interesting, specific application. I wish we could help you out with more data on that problem, but since it is kind of unusual, we can't.
Trying to get all pins connected with thick copper connections and getting the transceiver away from other heatsources will help, but that is probably no real news.

Regrads,
Till