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DEMO-5700D2Z: AD5421 unexpected current in loop

Hello,

I just bought DEMO-5700D2Z board and was verifying the functionalities. Last week I dumped the demo code provided, the current loop was varying from 4mA to 20mA as expected. But when I dumped the same code today, the current loop is as high as 80mA. I checked loading a different code (just to blink an LED) but still the current is above 80mA. I disconnected the rest of the circuitry from AD5421 and saw that it still took 80mA. Fault pin is high. I measured the voltage between COM and LOOP-, which was 4.47V implying almost 86mA is flowing through the 52ohm resistance inside the chip.

But I'm unable to locate what the exact problem is. Could anyone throw some light on what might the error be and what other parameters I should be checking?

  • Hi sora,

    Any current sourced from the loop supply will return via the COM pin and the 52R resistor inside the AD5421. For example another IC powered from the AD5421 regulator. This current will return to the loop and cause some errors.

    Can you describe what other connections you have made to the demo board outside of its schematic? i.e. how did you connect the LED that you mentioned above?

    Best regards,

    Rainier

  • Hi Rainier,

    Thank you for the reply.

    >Can you describe what other connections you have made to the demo board outside of its schematic? i.e. how did >you connect the LED that you mentioned above?

    I did not make any external connections. I used the LED connected to P2.1 pin on the board.

    I powered the board using 10V supply.

    Even if I download the demo code, it's consuming high current.

    >Any current sourced from the loop supply will return via the COM pin and the 52R resistor inside the AD5421. For >example another IC powered from the AD5421 regulator. This current will return to the loop and cause some >errors.

     

    To verify this, I removed the R11 resistor and powered 3.3V from external power supply. The current reading was 2mA. (I commented the AD5421 part in the demo code and used it).

    I suppose the AD5421 has gone bad, but how can I verify that and what could have possibly caused the IC to go bad?

    Thanks,

    Spoorthy

     

  • Hi Spoorthy,

    Yes there is a possibility that a component on the board might have been damaged.

    Can you check the ff for us to debug which component might have gone bad:

    • voltage at REGOUT pin of AD5421 (T5)
    • current going into R12 (you can compute this by measuring the voltage drop across R12)
    • current going into R20 (you can compute this by measuring the voltage drop across R20)

    Best regards,

    Rainier

  • Hi Rainier,

    Voltage at REGOUT pin of AD5421: 3.3071V

    Current going into R12: 0.0211V/10R = 2.11mA

    Current going into R20: 0.0025V/1R6 = 1.5625mA

    Unfortunately I hadn't ordered extra IC's to replace AD5421 and check. I have ordered now and may receive them next week.

    Also, I had another query about AD5421 which is not related to this particular issue: Can I simultaneously draw around 3.15mA from REGOUT, REFOUT1 Pin and DVDD output? (or) Is it that total current drawn from all the pins should be less than 3.15mA?

    Thanks,

    Spoorthy

  • Hi Spoorthy,

    It seems that the REGOUT of the AD5421 is still working and is still able to supply the current needed by its load.

    Can you also check if you shorted the -LOOP pin to COM with the board powered down?

    Let's us see what happens when you have tried replacing the AD5421 when you get them but it would be very challenging for us to troubleshoot now since you have dumped a different code as well as soldered/desoldered components from the board and may have damaged the components.

    Regarding your query, the total current drawn from REGOUT, REFOUT1, REFOUT2 and DVDD should be less than 3.15mA. Any excess will go back to the loop, for example you've drawn a total of 6mA from all of these pins, you will not be able to set the loop current less than 6mA.

    Best regards,

    Rainier

  • Hi Rainier,

    Yes, the voltage regulator is still working properly and there is no short between -LOOP and COM pins. Thus I'm unable to conclude what might have gone wrong. I will wait to receive new set of ICs and get back to you on this.

    Thanks for clarifying. So, I can draw max of 3.15mA at 12V REGOUT (other pins unused). I'm planning to then use a separate buck converter(say, ADP2360) to bring the voltage down to 3.3V and draw higher current of 8mA (assuming 80% efficiency). Is this possible or do you see any error in this?

    Thanks,

    Spoorthy

  • Hi Spoorthy,

    Connecting a buck converter to the REGOUT pin is possible. Just don't let the load current go back to the loop. I suggest using an isolator but that would decrease the efficiency to a point.

    Best regards,

    Rainier

  • Hi Rainier,

    If at all I draw more current, say 6mA, will it just shift the current loop from 4-20mA to 6-22mA or will it damage the IC in the long run? 

    And updating on the board status, I replaced the IC and it is working fine. But the current in loop is not stable, it keeps on decreasing slowly. What should I debug in order to fix this?

    Current at R12 is 2.16mA

    Current at R20 is 1.5625mA

    Thanks,

    Spoorthy

  • Hi Spoorthy,

    If you are drawing 6mA, you will only be able to set the loop current from 6mA to 20mA.

    Can you give more details regarding the loop not being stable? How are you measuring the loop current?

    Best regards,

    Rainier