ADN8834 Custom Design Issues

Hello, 

I am writing this because of a design issue I am having with the ADN8834. For background, I used the evaluation module to test the chip out first, and used the reference design in the datasheet to make a custom board for it. One thing to note is that the company I quoted for this circuit card had a lot of issues being able to manufacture the small form factor of the circuit, which could be part of the problem. I am writing this to verify my design first before I consider manufacturing issues. The voltage reference works, the Vilimcool seems to be constantly set at 1.6V indicating 1A even though I change the set temperature from the thermistor to either heat/cool. The voltage setting stays the same at ~2.3V which makes sense based on the datasheet. The TEC voltage seems to have roughly 1mV across the TEC and the OUT2 voltage seems to stay at 5.8V. That being said, I have roughly 3mA of current draw from the power supply which does not make sense. If you could help me verify my circuit that would be greatly appreciated. Attached are the schematic for the circuit and a portion of the board file that includes the TEC driver. 

Thanks and all the best, 

Nicholas TomaselloPDFPDFPDF

  • 0
    •  Analog Employees 
    on Apr 21, 2021 2:04 AM

    Hi Nick,

    Thank you for reaching out. I did a quick check of your schematic. The component values setting your ILIM and VLIM is good. Based on my checking, you set it to ~1.5A both in cooling and heating current. The TEC voltage limit on the other hand is set to ~4.75V. It looks good for me, I just need some further checking in your thermistor amplifier network. 

    Here are some questions/comments to clarify:

    - What is the level of your VIN? Are you operating now in closed-loop? with the thermistor and TEC module in place?

    - What are the individual output voltages of the TEC+ and TEC-? 


    Additional information so I can help you in the analysis:

    - What is the target temperature to be maintained?

    - What is the thermistor value you will be using? Can you also share the datasheet?

    -  What is the resistance of the TEC module that you will be using?

    Thanks,

    Noel

  • Hi Noel, 

    I was able to get the circuit to work. It turns out the low pass filter applied to VDD and EN/SY pins was only allowing 2.5V to send to those pins. I removed the 1k resistor in series with this and now everything works. I am not sure why that happened because I do want filtering at these pins, since it will improve the performance. 

    -Nick

  • 0
    •  Analog Employees 
    on Apr 29, 2021 3:28 AM in reply to Nick_T

    Hi Nick,

    It's good to here that you got it fixed. It's brilliant that you were able to identify that it is the low pass filter that is causing the part not to work properly. I understand that you want to reduce noise as much as possible. I am also not sure why that happened but my guess is, it is the combination of values of the R and Xc and the ripple frequency of your input that made this input voltage cut into half. 

    Thanks and regards,

    Noel

  • 0
    •  Analog Employees 
    on Sep 3, 2021 1:24 AM in reply to Nick_T

    1kohm for the low pass resistance to AVDD is rather large for the current.  The low pass filter in the reference design uses a 10ohm.  All the reference voltage divider networks are powered from AVDD through the low pass and you may not have enough headroom.  You may be able to use a 100 ohm resistor.