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DAC8531: DAC8531 low temperature

Part Number: DAC8531

DEARS.

Customer uses DAC8531.
It is not working properly in a low temperature test.
Setting DAC at low temperature is output 0V.
It is an abnormal operation.
The low temperature test temperature is -30 degrees.
SPI operates normally at low temperature(-30 degrees).
We are testing the chamber.
Raising the temperature will normally work.


What is the solution?

I would like a guide.

It is a circuit diagram of a customer.

Thank you

  • Hi Henry,

    Thanks you for your query. Could you please answer the following questions regarding the failure condition:

    1. What is the reference voltage when the failure condition occurs i.e. at -30 degrees?

    2. Do you also set the humidity inside the chamber in this condition? If yes, what is the measured value?

    3. Could you please check whether there is any possibility for condensation at this temperature and humidity?

    4. Do you see any possibility of short circuiting of the reference and/or output pins to the thermal pad or any other ground point on board? Please see if there are any traces of mist on the PCB traces and IC

    Regards,

    Uttam

  • Dear.Uttam Sahu

    Explain the problem in detail.

    The issue is a cold start item.

    It occurs when setting the value of DAC 8531 at low temperature.
    Output problems occur only at low temperatures.
    When the temperature at low temperature goes up, it is normal operation.
    When set to spi at low temperature, the output will be 0V.
    In this state, when the temperature is raised, the voltage of VC is gradually high (up to about 200 to 300 mV)
    It is the output is instantaneously normal.

    The reference voltage is 2.5V.
    It was confirmed that Reference Voltage 2.5V was maintained at low temperature.
    In normal operation, if reference_voltage is set to 2.5 V, the output level is 1.42 V.
    SPI operates normally at low temperature(-30 degrees).

    3. Humidity is 0

    4. It does not look like a problem caused by assembled state etc.

    What is the solution?
    I need help.

    Thank you
  • Hi Henry,

    We are trying to create the same setup in our lab. However, it wll take couple of days as we don't have the EVM with us currently. In the meanwhile, could you please test this condition on the EVM, in case you have it with you and see if the failure is reproduced?

    You have written humidity is 0. That doesn't seem to be a realistic case. Typically when you put temperatures that low, the humidity inside a chamber can go very high if it is not maintained and create condensation. It would be great if you could rule that condition out by properly setting the humidity to an approapriate value. When I mentioned about short circuit, i didn't mean it was to be due to assembly, but due to condensation caused by high humidity.

    Regards,
    Uttam
  • Hi Henry,

    Is there any update? I discussed with the design team and they suggested few more tests to find out the root cause:

    1. Could you please repeat the test on another PCB in order to rule out whether it is a part issue or not? As I had suggested earlier, you could also do it on an EVM if possible

    2. Could you check the current drawn by the system during the failure condition? If we can compare it with the current in normal condition, we will know whether the device is going to power down mode or not

    Regards,

    Uttam

  • Dear.Uttam Sahu

    We do not have EVM.
    I am waiting for applying EVM.
    The problem is the same for all boards during temperature test.
    We are continuing the test.
    Please help me.

    Thank you
  • Henry,

    Were you able to test the supply current? If you can do that, it will give us more insight into what might be the mechanism of failure, either power down or short-circuit. 

    Regards,

    Uttam

  • Dear.Uttam Sahu

    At Normal temperature, the current at the time of configuration is about 1 mA
    At low temperature, maximum 6 mA is consumed.
    We are continuing the test.
    Please help me.
  • Dear Henry,

    This current consumption at low temperature looks abnormal. We are trying to understand the root cause. In the mean while, could you please find the answers to following questions?

    1. Can you measure the supply currents in low and room temperatures when the output is in power-down state?
    2. What is the humidity in the chamber?
    3. Could you please share the schematics of the load stage? If you are not comfortable sharing it publicly, I can send you my email id. Please send it to that.

    Regards,
    Uttam
  • Dear.Uttam Sahu

    Are you testing DAC8531EVM?

    The humidity condition of the system should be up to 95%.

  • Dear Henry,

    At 95% humidity, there is a very high chance of condensation on the board that can lead to shorting of small-footprint device pins. I have seen this many times myself. In fact it is very difficult to measure humidity accurately beyond 90%. When they say 95%, at some locations inside the chamber, it may actually be 100%, especially near metal parts like device pins etc.

    One solution is to reduce the humidity during your testing, in case it is not part of your test specification itself.

    Another way is to cover the fine pitch pins by some kind of a sealant like conformal coating or silicone or epoxy glue. If you can do this coating on the DAC and the output oscillator, we can rule out the possibility of the condensation induced failure case.

    I am suspecting two points where shorts can happen: between pin 1 and 2 of the oscillator or pin 4 and thermal pad of the DAC. Both these conditions can lead to a failure condition we are observing.

    I have not been able to perform the test on the EVM yet. Sorry for that.

    Regards,

    Uttam

  • DEAR.Uttam

    when CAP(C1044) is deleted at low temperature, confirm the normal operation.

    I do not know the exact cause of CAP(C1044).

    I want to know how CAP(C1044) affects DAC8531.

    Please explain.

  • Dear Henry,

    I couldn't understand your point. Let me reiterate and please help me understand:

    1. When you remove the capacitor C1044 and test it under low temperature condition the system works fine
    2. You want to understand the impact of this capacitor on the DAC output.

    As per the datasheet, you should not use a capacitor of this large value in order to get a stable output. There is no mention about how it may behave under extreme temperature conditions. It will be difficult for me a get exact characterization data as this is a very old part. Let me check with the design team and get back on this.

    However, even at normal temperature capacitor of this value is not recommended to be used directly with the VOUT pin. You must use some compensation circuit in order to use it. A simple compensation circuit can be implemented by using a proper non-zero value for R982. However, you can use a better compensation circuit as the VFB pin of the output amplifier is available.

    Regards,
    Uttam