How to wire up the OE on SN74CBTD3384C level translator to make sure it works ok at startup

Joe Matthews83

Other Parts Discussed in Thread: SN74CBTD3384C

Hi Everyone --

I am coming from SW so please go easy on me :) I have a question about the SN74CBTD3384C. I am using this as a level translator and I see the datasheet has a note that says to use a pullup on the OE input to ensure it's always HiZ when it starts up. My question is, if I want to use this in a way in which it is basically always on otherwise *and* I don't have a pin to specifically connect this to which will pull this down, what else do I need to connect this pin to (GND? A cap to GND? Is this an RC circuit like 5V0->Res->OE->Cap->GND?). Any suggestions on values?

Thanks,

over 10 years ago

0 Chris Cockrill over 10 years ago

TI__Mastermind 46480 points

If you want to power up with the switch on then it is OK to tie it to gnd direct or you can tie it to gnd through a resistor.

The statement in the datasheet is just if you need the switch to powerup open.

0 Joe Matthews83 over 10 years ago in reply to Chris Cockrill

Prodigy 40 points

Ahh, ok, thanks Chris! (And thanks for the quick reply!) I suppose the only caveat to that is if there is potentially some oddball state that I'm not aware of on the pins at powerup that might have a cascading effect down the line?

Best regards,

0 Chris Cockrill over 10 years ago in reply to Joe Matthews83

TI__Mastermind 46480 points

correct, . It will passs on whatever is at the input.

0 Joe Matthews83 over 10 years ago in reply to Chris Cockrill

Prodigy 40 points

Thanks Chris -- I am using this to level translate a 5V input to a 3V3 output. What do I need to keep in mind to make this happen? Based on tables 4 & 5 in the datasheet, it looks like the level translation varies inversely to the load current. Is this correct?

Best regards

0 Chris Cockrill over 10 years ago in reply to Joe Matthews83

TI__Mastermind 46480 points

well, dont know why they show this. The output level vs current depend on the input driver. This is a switch so it is just a connection. the Ron vs current would make more sense.

The part will translate 5V to 3V fine as long as frequency is not over 10Mhz.

any buffer that has 5V input tolerance would do the same thing. SN74LVC827

0 Joe Matthews83 over 10 years ago in reply to Chris Cockrill

Prodigy 40 points

Thanks Chris! This makes a lot more sense. I've tested this now that the parts have arrived and I can confirm what you indicated. With 5V0 I'm getting 3V3 on the other end and a very nice looking signal working in either direction (which is what I was after). :) Thanks for taking the time!

Best regards,

Logic

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How to wire up the OE on SN74CBTD3384C level translator to make sure it works ok at startup