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SN74CB3Q3257: The VIH level could support 1.8V or not?

Kay Chen53
Expert 2225 points
Part Number: SN74CB3Q3257
Other Parts Discussed in Thread: SN74CBTLV3257,

Hi there:

Our customer original used the SN74CBTLV3257 this one but found the VIH(min) =2V @ VCC = 2.7 V to 3.6 V condition.

So we'd like to purpose the SN74CB3Q3257 for support and still found the VIH(min) =2V @ VCC = 2.7 V to 3.6 V condition.

But notice the 8.3 feature description :

"The VCC operating range is from 2.3 V to 3.6 V and the data I/Os support 0- to 5-V signal levels of (0.8-V, 1.2-V, 1.5-V, 1.8-V, 2.5-V, 3.3-V, 5-V)."

from this section, I'd like to double confirm:

1. Does VIH = 1.8V works or not?

2. If not, is there any possible solution could suggest for customer replace it? (1-of-2 Multiplexer/De-multiplexer) 

3. Is there special reason for the VIH(min) =2V this condition? 

B & R

Kay

  • 0
    TI__Intellectual 2900 points
    Hi Kay,

    VIH is for S and OE pin, so VIH has nothing to do with A pins and B pins.

    Section 8.3 says data I/O support 0 ~ 5V, meaning A pins and B pins can be from 0.8V, 1.2V, 5V etc. However, S pin should still comply VIH = 2V requirement.



    When the supply range is 2.7V to 3.6V, we actually assume customers will typically use 3V or 3.3V power supply. We designed with +/- 10% margin.

    To be specific,
    2.7V = 90% x 3.0V
    3.6V = 110% x 3.3V.

    In most digial interface, VIH and VIL typically follow 20-80 rule ( a few companies use 30-70 rule). VIL is 20% of supply voltage while VIH is 80% of supply voltage.

    When VCC = 3.0V, VIH = 2.40V, VIL = 0.60V
    When VCC = 3.3V, VIH = 2.64V, VIL = 0.66V

    SN74CBTLV3257 and SN74CB3Q3257's VIH = 2.0V and VIL = 0.8V. Apparently, TI wants to give customers extra margin in addition to 20-80 rule.



    Would you like to share a bit more about your customer's design? I would like to know why your customer wants to have 1.8V VIH(min) supported.

    If it is a voltage shifting issue, there are two ways to solve it.

    1. Use Open Drain I/O to drive our MUX (the most economic solution)

    2. Insert a 1.8V to 3.3V level shifter, SN74LV4T125PWR for example.