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DAC7760: Overheating, thermal alarm - automatic shutdown or user shutdown?

Simon Wyss
Prodigy 50 points
Part Number: DAC7760
Other Parts Discussed in Thread: DAC8760

We are using the DAC7760 in hot environment: up to 70°C ambiente Temperature with 24V single supply.

I have a question concerning the overtemperature alarm:
Should the user shut down the device, if overtemperature alarm is rised?
Or does the device protect itself before it gets destroyed?
Does it have a thermal protection?
Is the output current reduced in case of over temperature?

We found no information in the data sheet concerning this question.
I look forward to hear some advice.

I did power and thermal calculation. The device temperature should be fine (within specified range).
However, we had many defective devices (6 out of 20).

Calculated Power dissipation of our use case:

Current Output: 0..20mA, 300R Load Resistor
20mA * 300R = 6V, Power dissipation in DAC: Pdiss_output = (24V-6V) * 20mA = 18V * 20mA = 0.36W

Internal Linear Voltage Regulator 4.6V output, @ 10mA Load: Pdiss_4V6 = 19.4V * 10mA = 194mW ~ 0.2W

Total Power Dissipation: 0.56W

Thermal Resistance: (RHA, VQFN, 40 Pins)
Junction-Ambiente: 32.9 °C/W ~33°C/W
Junction-Case-Top: 17.2 °C/W
Junction-To-Board: 7.5 °C/W
Junction-Case-Bottom: 1.4 °C/W

Worst Case: 0.6W * 33°C/W = 19.8 ~ 20°C uplift vs Ambiente
70°C + 20°C = 90°C

Abs max Temperature Rating:
Junction: 150°C
Operating Temperature Range (Ambient ?): -40...125°C

  • 0
    TI__Expert 5820 points

    Hi Simon,

    DAC7760 will not take any action when the thermal alarm triggers so this will have to be monitored by reading the status register. The alarm pin will go low if any alarm condition is met and then the status register can be read to indicate which alarm has triggered. The MCU would be responsible for disabling/reducing the output.

    I agree with your power dissipation and temperature estimates. I have provided a thermal image of DAC8760 EVM driving a 250Ohm load at 20mA with a 24V supply. This was at room temperature and you can see around a 17C rise which agrees with calculation. PCB parameters and enclosures can greatly affect the thermal resistance.

    \

    When you say you have defective devices I assume this means overheating. How hot are the devices getting? Can you provide a schematic for review?

    Thanks,
    Garrett

  • 0 in reply to Garrett Satterfield
    Prodigy 50 points

    Thank you for the support!

    We will monitor the alarm and protect the device by software.

    However, considering the thermal calculation, I think the cause of the defectives was not the over temperature. More reasonable an short over voltage (spike, overshoot) during power supply ramp up could be the source of the defectives. 

    See thread: 

    Therefore, I added a 24V TVS Diode on the AVDD and a 5 Ohm Resistor in series in front of the capacitor on the AVDD pin. (This solution was mentioned in the thread above, to decrease the power supply ramp up speed and reduce over shoots.