• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 45 subscribers
  • Views 1758 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DAC7311: Maximum allowed permanent current through ESD diodes on the SPI bus...

Kris Verbeeck
Prodigy 100 points
Other Parts Discussed in Thread: DAC7311

Hello all,


Made a bit of a mistake. I have a ADUM5401 digital isolator where the output voltage was changed to 5V rather than 3V3 rather late in the design stage. The ADUM feeds an SPI bus to a TI DAC7311. The supply voltage is not so much a problem, but I didn't take into account the signal levels for this (stupid, I know). As a result, if the ADUM output is "on", it rams about 15mA through the ESD diodes on the input of the DAC. To make it even worse, one of the signals is a CS which is almost always high. So, a lot of power wasted and, I assume, an unhappy DAC7311.


The simpelest solution would obviously be to limit the current with a series resistor. I currently have this implemented with a 1k resistor which limits the current to about 1mA or so. From what I can find, this is a safe value for most/all ESD diodes out there. This is, however, a design that is supposed to opperate in harsh conditions for a long time and so my best guess isn't exactly enough for this project. I know some datasheets that list max permissible current through the ESD diodes, but the DAC7311 sheets don't mention it.


Is there a TI bloke out there that can close the loop on this one?

  • 0
    TI__Genius 10615 points

    Hi Kris,

    While limiting the ESD diode current to 1 mA via a series resistor is probably safe, you will need to increase this resitor value until the digital pins are within the Abs. Max. specification. The digital pins should be < VDD + 0.3 V. This is specially true for your application. In order to maintain the performance of the device over a long time.

    I understand that these kind of slip-up occur. To be thorough in our solutions, can I ask you a few questions? Maybe we can find an alternative solution.

    How long is this expected time?

    What is the DAC connected to? If it is an amplifier network, we might be able to re-size the MAX reference voltage and the gain of the amplifier network to try to achieve better results.

  • 0 in reply to Eugenio Mejia
    Prodigy 100 points
    Thanks for the reply! How long is the expected time? Pretty much forever, or at least 15+ years. There are a number of app notes for using ESD diodes with a permanent current (SLAA689 from you guys, Analog Dialogue 49 from AD and I guess a few others). So it would seem an acceptable tactic but then you need to know the max current which is supposed to be in the datasheet. But isn't. If there is no way of getting someone to put a number on it I will have to add buffers. Even though messing around with references etc might be an option in theory, the road of least resistance would probably be to add the buffer.
  • 0 in reply to Kris Verbeeck
    TI__Genius 10615 points
    Hi Kris,

    I will be having a discussion with the designers tomorrow regarding this issue. I will give you another update tomorrow.
  • 0 in reply to Eugenio Mejia
    Prodigy 100 points

    Hi,

    Just checking in, is there any progress on this?

    Kris Verbeeck

  • 0 in reply to Kris Verbeeck
    Prodigy 100 points

    Hi,

    Sorry to keep hammering this same nail again, but I'm doing Rev.B of this thing and I'm running out of things to clean up besides this. Any news?

    Kris

  • 0 in reply to Kris Verbeeck
    TI__Genius 10615 points
    Hi Kris,

    I was able to talk to the designers. Sadly I don't have any new info for you. The answer remains the same. While 1 mA is very little current and there is small chance for something to go wrong. We cannot guarantee a part in a system that violates the Abs. Max. specifications.