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SN74LVC125A: Need VOHmin >3,15 @IOH = 0,7mA

Part Number: SN74LVC125A

We will be supplying the device with 3,3V. The VOH is the critical parameter and we can only “afford” to let the output voltage drop 0,15V from the supply, so with a nominal 3,3V supply, we need VOHmin >3,15 @IOH = 0,7mA

 

Reading the datasheet, I find the following information :

 

 

It seems that @ 0,1mA, we have a “risk” of loosing 0,2V, and for 12mA it is 0,6V – how “conservative” are these figures ?

 

Another curve is found as well :

 


 

Looking at this curve, it seems that the output impedance around 3,3V and 1mA is 0,5V/18mA = 28ohms and VOH @IOH =-1mA is only a fraction below the supply voltage of 3V.

 

Can we safely use the curve above for determining the output “ON” resistance for the device ?

Do we risk loosing 0,2V@0,1mA as stated above or is this related to supplying the device with 1,65V only ?

Should the part of the curve exceeding VOH = 3V be interpreted as the device is being driven from the outside as shown on figure 8 in the datasheet (although not recommended)?

  • We meet again 007 -- I didn't expect to see you again so soon!

    The actual performance of the device is better than the guaranteed limits in the datasheet, by definition -- many of our customers know this and use it to their advantage, but there is risk involved in doing this. TI only guarantees what you see in the datasheet, so if that is beyond what the customer can support, there's nothing I can really do to help them. We must spec for the absolute worst case across PVT, regardless of the fact that a "very weak" or "very strong" process corner will probably never make it to the market -- and that's where the datasheet values come from.

    *To be clear for your question, 1.65V specs are for the 1.8V node -- from 1.65V to 1.95V, with 1.65V being the worst performance.

    That being said, we can talk about the expected real-world performance of the LVC family of logic. These devices have typical output impedance around 50 ohms at 1.8V supply. With a 0.7mA load, you can expect ~40mV of drop at the output -- ie if you factor noise and typical operation into the mix, the output will be ~Vcc or ~gnd. I'll attach a tina simulation here to let you mess around with values:

    SN74LVC1G17_voh_example.TSC

    The curve shown is good for typical operation. If we're talking about mass production and across temperature operation, then the datasheet specs are what to go by.

    Please let me know if I can be of further assistance.