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LMK04828: Why the 560ohm resistor was added to the latest datasheet

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Part Number: LMK04828

Hello,

My customer found that the 560 ohm resistor in LVDS/HSDS output was added to the lastest data sheet, which was not included in the previous data sheet(refer to the attached old data sheet).

So, they think TI recently discovered an error in LVDS/HSDS and added this to the data sheet to fix it.

Is this right? Please let us know what caused 560 ohm to be added to the latest data sheet.

Thank you.

JHlmk04826.pdf

  • Hi JH,

    We discovered an implementation detail of the LVDS/HSDS driver that causes difficulty when the LVDS/HSDS format is first powered up after format selection. In cases where there is no DC path for current flow between the P/N terminals of the output, sometimes the clock will take several hundred cycles to start, or may fail to cleanly start; worst case, the output clock will not be generated. Placing the 560Ω resistor across the P/N terminals before any AC-coupling ensures that there is a DC path for current flow on startup, which ensures LVDS/HSDS outputs can always power up and begin oscillating normally.

    There is low risk to existing designs which have already been validated, which may be lacking a DC path across the P/N terminals while still using LVDS/HSDS formats with AC-coupling capacitors. Prolonged delay before clock startup will not affect the divider or digital delay timing, and failure to oscillate is extremely rare and specific to use cases involving AC-coupled LVDS/HSDS and differential band-pass filters. If the issue hasn't appeared before for a given schematic and layout, there is essentially no risk of failure to oscillate.

    Regards,

    Derek Payne

    Texas Instruments