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SN74AVC4T774: Lowering VCCA voltage

Den L
Intellectual 970 points
Part Number: SN74AVC4T774

Hello, 

I have a small question for the seasoned TI engineers :) It is not totally related to SN74AVC4T774, but since I am using it, i thought I will ask it here. 


I am using the SN74AVC4T774 for voltage translation from 1.8 to 3.3 for my UART signals, see image below. The IC on the left has 3.8V power supply and it conveniently generates 1.8V of output voltage supply for its digital signals, such as UART. The IC on the right (It's a Module) requires 5V power supply and has 3.3V UART interface, but it doesn't have such convenient 3.3V output power supply pin. Therefore I though of just taking the 3.8V from the left IC and lowering it with a Schottky diode. However it might be a bad idea since the forward voltage changes with current consumption and I have no idea how much SN74AVC4T774  will consume during UART activities, or when UART is idle. So basically the VCCA might be not stable enough, do I understand it right? or is the current consumption on VCCA stable enough and I am overthinking it? 
Another option could be to use a Zenner diode as a voltage regulator...

Anyway, how would you solve this problem? Any tips are welcome! Even Schottky diode suggestion are welcome ;)

Kind Regards,

Den

  • 0
    Guru 245050 points
    The power consumption of CMOS devices is very low when idle and rather high while switching, so a Schottky diode will not work (and 0.5 V wold not be easy to hit anyway).

    You might try adding a dummy resistor between 3.3V and GND to get a larger, constant power usage, but it would be a better idea to use a 'real' voltage regulat (a linear LDO, or a shunt regulator (low-voltage Zeners suck; use a TL341 instead)).
  • 0 in reply to Clemens Ladisch
    Intellectual 970 points

    Thank you Clemens for your answers!

    I see, so I guess a simple LDO is the easiest way.