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TS3DS10224: VIH low voltage recommendations

Part Number: TS3DS10224
Other Parts Discussed in Thread: SN74AXC4T245

For a customer the control inputs to the TS3DS10224 are being driven by an IC that only supplies 1.8V as logic high. The TS3DS10224 datasheet recommends 0.75*VCC as the minimum VIH, but their voltage is well below that with VCC = 3.3V (1.8V vs 2.475).

They would like to understand the risks associated with violating the recommendation. Can you advise regarding the potential problems that would occur with regular, prolonged use of 1.8V on SA0, SAI, SBI? (The multiplexer control inputs).

  • With such a low voltage, the input might not even switch high. But you will certainly get high cross currents; see [FAQ] How does a slow or floating input affect a CMOS device?

    This requires a level shifter, e.g., SN74AXC4T245.

  • Hi Emil,

    If you cannot meet the VIH requirement (0.75*VCC), then there is no guarantee the device will be able to switch between channels (since the control inputs may never register 1.8V as a logic high). As Clemens mentioned, a level shifter would be a suitable solution to meet your 1.8V needs, otherwise this application will not work as intended.

    Thanks!
    Bryan

  • Updated question from the customer:

  • Thier current plan is to use a 3.3V pull-up, and make the other IC's outputs high-z in order to set the MUX input high. They'll set the other IC to output low to drive the MUX input low. 

    They realize the current issue is a real concern, but, for Vih, can you share any characterization plots (or similar) to give us a sense for how close we are to not working? We have a few boards that we could potentially rework, but it's not a trivial job so we're weighing our options.

  • Hi Emil,

    We do not have the characterization plots available, however, the way the VIH is specified is that this value guarantees the device will turn on at at least that given voltage across temperature and process variation. Given that, there would be no guarantee at any voltage below this value that it would turn on every time. Even if I did have the characterization plots, this would give a false sense of security as the spec is defined in combination with our process variation which can fluctuate a certain amount and would not be publicly available.

    We do our best to make these specifications as tight as we can, so I would advise to stick to the provided datasheet numbers as they were calculated with all aspects of how this device operates and how it is manufactured in mind.

    Thanks,

    Bryan