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Schmitt Trigger SN74LVC1G14 VT+ Query

David Cooke
Prodigy 70 points
I read with interest  "Understanding Schmitt Trigger" app note: http://www.ti.com/lit/an/scea046/scea046.pdf
 
I do however have a query on the VT+ spec. In the document it states:
 

1. On the input rising edge the part will be guaranteed to switch between (Vt+ min) and (Vt+ max).

2. In the figure above, the input levels Vih and Vil must be greater than (VT+ max) and less than (VT– min) to insure the part will switch

3. We specify the part will switch on the rising edge between (VT+ min) and (VT+ max)

To me there seems to be a contradiction between statements 1,3 and 2. For example, I want to use a schmitt trigger as a 3.3 - 5V translator. The input to the schmitt trigger logical '1' worst case min is 3.1V and the schmitt trigger VT+ max = 3.035 and VT+min - 2.42. In theory then there is not a problem, but the margin is small If VT+ max, is the number to use.

I'm actually confused and do not see the point in specifiying VT+ max. Surely like a logic gate, all that matters is the "VIN H min" or VT+ min?

Can anyone offer any advice please?

 Many thanks

David

  • 0
    TI__Mastermind 46480 points

    Hi David

    The part will switch on the rising edge between VT+min and Vt+max. So in order to guarrantee a switch your high level must be greater than or equal to VT+max.

    Your low level must be equal to or lower than VT- min in order to guarrantee a switch.

    This statement should read like this to be clearer

    2. In the figure above, the input level Vih must be greater than (VT+ max) and Vil must be less than (VT– min) to insure the part will switch

     

  • 0 in reply to Chris Cockrill
    Prodigy 70 points

    That's great thanks. Can I ask if there is any trend data on the VT+ thresholds - or thresholds in general? Of course one owuld design to meet the datasheet specs, but the product has been designed and I'm trying to determine how much of an issue I have. Worst case (from datasheets, voltage rails, temperature etc) I have 40mV margin. On the bench I see 400 mV (at extended temperatures). I'm just wondering if I got lucky with this part. I have tested 4 from the same batch with the same result.

    Thanks.