0
\$\begingroup\$

When browsing through available bidirectional level translator ICs, I always see the condition that reference voltage on one side of the translator must be strictly less than voltage on the other side.

For example, ADG3300:

For proper operation, Vcca must always be less than Vccy.

Where does this limitation come from? Aren't bidirectional level translators symmetrical in their internal structure?

Also, are there any level shifters without this limitation? (or is it possible to create such?)

\$\endgroup\$
7
  • \$\begingroup\$ Haven't looked at this one, but it often becomes clear when looking at the equivalent circuit provided with lots of datasheets when you trace out where those two "meet" and sometimes one is at some source while the other is at some drain or similar things. \$\endgroup\$
    – PlasmaHH
    Aug 12, 2015 at 10:27
  • 2
    \$\begingroup\$ bidirectional != symmetrical. Take a look at the Theory of Operation starting on page 15 of that datasheet, where it explains "The logic level translation in the A Y direction is performed using a level translator (U1) and an inverter (U2), and the translation in the Y A direction is performed using the inverters U3 and U4..." \$\endgroup\$ Aug 12, 2015 at 10:27
  • \$\begingroup\$ @PlasmaHH Yes - datasheet pretty much explains it all, thank you. The question on availability of symmetrical level shifters still remains, though. \$\endgroup\$ Aug 12, 2015 at 10:41
  • 1
    \$\begingroup\$ @RogerRowland - many thanks, this closes the 1st part of a question. \$\endgroup\$ Aug 12, 2015 at 10:42
  • \$\begingroup\$ Would you also expect a mains transformer to be bidirectional i.e. whichever winding you put 230V on always produced 12V on the other? \$\endgroup\$
    – Andy aka
    Aug 12, 2015 at 10:47

1 Answer 1

0
\$\begingroup\$

It's likely because usually the designer is specifying the two supply voltages and they are thus known a priori. Since there is likely a cost in performance and/or die size to making a symmetric level converter, the majority of the market can be well served by types with specified Vcca > Vccb or whatever. For example, the common bidirectonal level conversion method using a discrete MOSFET (from here):

http://www.hobbytronics.co.uk/mosfet-voltage-level-converter

..will not work if you swap the supply voltages.

There are available converters with symmetric supply voltage limits, for example, the TI TXB0304, which could come in handy if you needed to program the supply voltage or have it supplied externally in order to deal with customer requirements (the alternative might be to operate all your circuitry at the lowest or highest voltage possible, or to use two converters and similarly set an intermediate supply voltage).

\$\endgroup\$
1
  • \$\begingroup\$ "if you needed to program the supply voltage" - this is exactly my case. Thank you for your answer. \$\endgroup\$ Aug 12, 2015 at 16:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.