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DAC121S101 inputs are high impedance when powered down?

Dennis Seitz
Prodigy 70 points
Other Parts Discussed in Thread: DAC121S101
We’re using the DAC121S101 in a spaceflight application.
We recently realized that our power supply sequencing could apply 3.3V through a 100 Ohm resistor to the DAC121S101 SYNC input on pin 7 while the VA on pin 1 is still at zero Volts.
We understand from the datasheet that the input voltage must not exceed the power supply by more than 0.3V, and the input current must be limited to 10mA. But from what we've observed, we think that might not apply to power-down conditions. We've observed < 1mA into the SYNC input when the pin1 voltage is zero and the SYNC input is tied to 3.3V through a 100 Ohm resistor. This was unexpected. We expected that the SYNC input would conduct as much as 30mA.
This device is designed for several power-down modes of operation and has power-down logic. We suspect that the inputs might be in a high impedance state by design when the power is off.
Can a TI rep tell us if that input is in a high impedance state when the power is off, by design? If so, can you confirm that it's safe to apply 3.3V high logic levels to the SYNC, SCLK and Din inputs of the DAC121S101 while it is powered off?
We need this as an official answer from TI. We tried a direct email to support@ti.com but received an automated reply that informed us that we must post our question here.
Thanks.
  • 0
    TI__Genius 16890 points

    Hi Dennis,

    The official answer is what is stated in the datasheet, the abs max rating for any pin is VA + 0.3.

    Here is more information on what is happening on the SYNC pin.  There is a internal diode to ground.  If the voltage on this pin goes more than about 0.3V beneath ground it will start conducting.  There is no diode between this pin and VA, which is why you are not seeing the current go up when the voltage is at 3.3V and the power is off.  The voltage on the SYNC pin is probably going into a gate of a transistor so the current will be low.  The danger is that when the voltage becomes too high you can damage this transistor.  3.3V probably will not damage it but this is not certain. 

    For best reliability you could install an external Schottky diode between this pin and VA.  Then when the voltage is more than 0.3V above VA the Schottky diode will start to conduct and protect the SYNC pin.  You would also be good to increase the series resistor so that the current stays below 1mA when the diode is conducting. 

    Mike

  • 0 in reply to Michael Stout
    Prodigy 70 points

    Thanks for the official reply, Mike.

    Please confirm that the diode to ground has it's anode connected to ground, and is reverse biased when we apply +3.3V to the input.

    To minimize the rework on our spaceflight board, we'd prefer to only increase the series resistor to 499 Ohms, but not install the diode you describe.

    However, this would result in 3.3V still appearing at the SYNC pin if no current flows, exceeding the VA+0.3V maximum requirement. Would this still pose an unacceptable risk to the input transistor? In that case we would have to install retrofitted diodes in many places on our flight board, which could be difficult.

    Sorry, I have to ask. Retrofitting diodes on a flight board is a serious step and I need to be sure it's absolutely necessary.

    Thanks.

  • 0 in reply to Dennis Seitz
    TI__Genius 16890 points
    Hi Dennis,

    That is correct, the anode is connected to ground and the cathode to the SYNC pin.

    Putting VA at ground and a DC voltage of up to 6V on the SYNC pin did not show any diode connected between the SYNC pin and VA. The process is has an abs max of 6.5V, so the 3.3V is probably ok. The main reason for an external diode between an input and VA is to keep the upper ESD diode from turning on, which this part does not have.

    Another thing to think about is if the signal is ever changing when the power is off. If there are edges on the SYNC pin with no power on the VA pin, there is the possibility of momentarily turning on a parasitic device as the signal is changing. This is something I can't see on the curve tracer. If this should happen the current during this time needs to be kept low, the spec say 10mA, but closer to 1mA is better. This is where the series resistance is helpful, to minimize this current if there are any signal edges. The 499 resistor would give a possible momentary 6.6mA as the signal is changing. I don't know if this would happen, but it is just a possibility I wanted to mention.

    Mike
  • 0 in reply to Michael Stout
    Prodigy 70 points
    Thanks again Mike. My next question was going to be what the process max voltage is, so you answered that already.

    We'll have to hold a review meeting to propose and authorize a solution. We'd like to say that Texas Instruments has confirmed that the process max is 6.5V, and that 3.3V should not pose a risk, which over-rules the datasheet. Is that acceptable?

    We'd propose that the solution is to simply increase the series resistors to 499 Ohms and ensure that no edges are present on the lines while VA is at ground.

    Can you share a schematic of the input circuit with us, by any chance? It would be helpful for the review.

    Thanks,
    Dennis
  • 0 in reply to Dennis Seitz
    TI__Genius 16890 points
    Hi Dennis,

    I looked through the documentation I was able to find and cannot find a input schematic for the part. From the tests I tried in the lab we know there is a diode between SYNC and ground, but no diode between SYNC and VA. The block diagram on the first page shows it going into some digital circuitry after that. I would imagine that it is probably similar to the figure on page 5 of the datasheet but I have not been able to confirm that.

    I agree that increasing the resistor is a good step. The process max is 6.5V and 3.3V most likely will not cause damage but I can't guarantee that and override the datasheet.

    Mike
  • 0 in reply to Michael Stout
    Prodigy 70 points

    Thanks, Mike. I'll pass on to our review board that TI makes no guarantees or exceptions to the datasheet.

    Dennis

  • 0 in reply to Michael Stout
    Prodigy 70 points
    Mike, we think we've identified a more attractive solution:

    We propose to connect a Schottky diode between the 3.3V supply (anode) and the 5V supply being used for the DAC VA (cathode). When the 3.3V supply comes up before the 5V, the DAC VA pin will be pulled up to 3.0V, and the 3.3V powered logic output applied to the -SYNC pin will be within the VA+0.3V maximum acceptable range. After the 5V comes up the diode will be reverse biased and the circuit will function normally.

    Do you see any drawbacks to this approach? In our estimate, it's effectively identical to putting a Schottky diode to VA on every one of the -SYNC pins (we have 8 devices), but instead requires only one diode, and minimal wiring.

    Thanks again,
    Dennis
  • 0 in reply to Dennis Seitz
    TI__Genius 16890 points
    Hi Dennis,

    Yes, that is a good solution. It should work as you described and will keep the SYNC pin within the abs max rating.

    Mike
  • 0 in reply to Michael Stout
    Prodigy 70 points
    Great, thanks.

    Dennis
  • 0 in reply to Michael Stout
    Prodigy 70 points

    Mike,

    The Schottky diode we have in stock and approved for flight has a 0.4V to 0.5V drop across our operating temperature range.

    Therefore the voltage between -SYNC and VA could be as high as 0.2V over the "absolute maximum" 0.3V spec.

    Would you be able to authorize this, since the process has a 6.5V reverse breakdown rating? Or will we have to identify a Schottky that will have </= 0.3V forward drop across our temperature range?

    Thanks again,

    Dennis

  • 0 in reply to Dennis Seitz
    TI__Genius 16890 points

    Hi Dennis,

    I can't authorize anything that contradicts the datasheet since the datasheet is our contract with the customer.  However, as you mention, with the 6.5V breakdown rating, I don't think it will be a problem being 0.2V over, especially as no current is being drawn into this pin, you have a series resistor, and the time that this will occur is of a very short duration.

    Mike