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SN75115: Minimum VID to guarantee a high or low output

Cedrick
Expert 8270 points
Part Number: SN75115
Other Parts Discussed in Thread: SN75114, THVD1550

Hi, 

Good day. I hope you are doing fine. 

Our customer wants to use the SN75115 however he need guidance on the VID and the pull up/ pull down resistor for their application. Kindly see the inquiry below. 

"The datasheet seems to indicate a min VID of 0.5 VDC which seems high to me. Latter on in the data sheet Figure 7 seems to indicate somewhere below 0.1 VID. We are going to add pull ups and pull downs in case of a cable disconnect but the 0.5 is driving the pull up and pull down resisters to 585 ohms each with the 130 termination resister connected. I would expect around 1k for the pull ups and pull downs. Am I missing something. If not we are using this chip with the mating driver chip SN75114. What would you recommend for pull up and pull down resisters. This is not high speed so the 130 could be removed."

I would appreciate if you could help us provide the information needed. 

Stay safe and have a great day. 


Regards,

Cedrick

  • 0
    TI__Guru 62920 points

    Hi Cedrick,

    Agreed that the VID requirement of 0.5 V is relatively high, but keep in mind this IC was designed in the early 1970s. :)  From Figures 7 and 8 in the datasheet you can see the typical VID threshold and how it varies with different conditions.  It looks to nominally be closer to 0 V, but with higher VCC and lower operating temperature it can shift - this is likely what is driving the 0.5 V spec limit.

    Given the limited data available, though, I would recommend designing conservatively based on the existing datasheet limit.  This would unfortunately mean use of low-valued pull-up/pull-down resistances or an adjustment to the differential termination resistance used (either to omit it or to increase its value so that the pull-up/down resistances could be scaled up accordingly).  Or, another option to consider would be a device with tighter VID specifications.  Dual receiver devices are a bit rare, but single-channel options are often available in small package sizes.  An example would be THVD1550, which is available in 3 x 3 mm VSSOP and has a "VIT+" threshold below 0 V so that systems could operate without external pull-up/down resistances or with very high-valued ones.

    Regards,
    Max