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AD5753 VDPC– current draw issue

Hi everyone,

we designed an analogue IO module with 16 independent AIO's, divided in 8 groups of 2. Each group of 2 is supplied by an isolated DC converter providing +-20V, 42mA per rail. When testing the board we had some converters magically fail after a while. We use both current and voltage output modes, mixed between all the channels.

After a little research we found that the AD5753's are using considerably more current on the negative rail then on the positive.

If the DCDC_MODE register is set to DPC Current mode, output set to 24mA (through a 500R load), current draw on the positive rail is 19mA. If we set the output to -24mA, the current draw on the negative rail rises to 60mA, then drops to 36mA with a reduced current output (-21.78mA) and reduced Vdpc-.

If the DCDC_MODE register is set to DPC Voltage mode, without any output set, current draw on the negative rail immediately rises to 34mA, whilst the positive rail only increases by 2mA.

In either mode, if we leave it on long enough it goes into a mode were it doesn't even reach its full negative output range. We looked at the board with an FLIR, the DAC was about 65 degrees Celcius. When spraying "Freezer spray" on it, we can get it "working" again. That means, Vdpc- voltage ar the right level, output current at the set current, but still to much current consumption from the negative supply rail. 

We clearly see different behaviors between the Vdpc+ and Vdpc- rails. We took some pictures with our scope:

  • Yellow = Vdpc+
  • Green = Vdpc+ Swithcing Node
  • Pink = Vdpc-
  • Blue = Vdpc- Switching Node

Image 1: DCDC_MODE register in DPC Voltage mode, no output current or voltage: 

DPC Voltage Mode

Image 2: DCDC_MODE register in DPC Current mode, 24mA output: DPC Current Mode +24mA out

Image 3: DCDC_MODE register in DPC Current mode, 0mA output

DPC Current Mode 0mA out

Image 4: DCDC_MODE register in DPC Current mode, -24mA output

DPC Current Mode -24mA out

Image 5: Used schematic. VIout_0_CHx and VIsns_0_CHx are connected at the connector:

AOM Circuit

As you can see in image 3, even with no output, the Vdpc- has to deliver more current since the duty cycle of the Vdpc- switching node is greater. When setting the output to -24mA (image 4), the Vdpc- works at first, but as the time goes, current consumption increases, switching node oscillations slowly disappear upto the first peak, then Vdpc- decreases and output current goes to ~21.78mA.

As we ruled out anything else we're stuck with a solution for this problem. The extra current draw has to be directly on the Vdpc- rail, but there is nothing else connected to it except the AD5753 itself. If it was an extra load somewhere on the output, the current output wouldn't work at all. How can we fix this?

Best Regards,

Daniël Kunst

Hardware Engineer @ 3T Electronics

  • Hi Daniel, 

    I have some questions to better understand your setup.
    1. Can you provide the settings for your DAC Config, DCDC COnfig 1, DCDC Config 2 Registers?

    2. Were you able to follow the suggested programming sequence on p52&53 of the datasheet? as well as the suggested wait times.

    we designed an analogue IO module with 16 independent AIO's, divided in 8 groups of 2. Each group of 2 is supplied by an isolated DC converter providing +-20V, 42mA per rail.

    3. Do you mean that there are 2 AD5753's sharing on one isolated DC converter? 

    4. in the tests you did above, are you testing only one AD5753 at a time?

    5. When you test for the DPC Voltage mode, did you do it from Power on or did you change from Current mode to Voltage mode?

    6. How did the out setting transition from 24mA to -24mA? is there a step in between or straight FS to -FS? is this the same response when you initially set to -FS (-24mA) upon power up?

    7. What's the status of the VPIN and VNIN in your tests? I hope to see the voltage waveform if it is still consistently @20V. 

    Best regards,

    Ian

  • Hello Ian,

    thanks for your reply. Hereby the answers on your questions:

    1: Register settings:

    • DAC Config:
      • INT_EN enabled
      • OVRNG_EN enabled
      • RANGE 0011 / 1001 dependent of output mode.
      • After DCDC registers are written, OUT_EN enabled
    • DCDC Config 1: 
      • DCDC_MODE in  DPC Current mode (we also tested DPC Voltage mode, see image 1)
      • Other settings default
    • DCDC Config 2:
      • DCDC_NEG_EN enabled
      • Other settings default

    2: Registers are written in the correct order. We have no WAIT loop between them, we are using the "IS BUSY_3WI = 0?" branch of the chart on page 53.

    3: Exactly, 2 AD5753's share one isolated converter. Each board has 8 isolated converters with 2 AD5753's. For these tests we removed the isolated DC converter and supply the circuit directly with a benchtop supply.

    4: In the tests above I tested AD5753 at a time. The other one was powered, but not outputting anything. This 2nd AD5753 is not included in the numbers.

    5: We tried both options. When powering up the AD5753 the first time it is configured for DPC Voltage mode, after that we switched to Current mode and back with the same results. We can see the selected option is enabled on the scope.

    6: We tested the current and voltage output with a sweep from -FS to +FS, this gives us for both settings a straight line without any steps (other than the auto ranging of the multimeter)

    7. VPIN and VNIN are supplied by a bench supply (limited to 100mA per rail) since we didn't want to blow up more converters. This provides a stable supply for the circuit.

    Best regards,

    Daniël

  • Hi Daniel,

    The EPAD on the AD5753 should be connected to AVSS or left floating. Looking at your schematic the EPAD is connected to ground. Can you confirm this?

    Valerie

  • So a colleague of mine found the mistake. The Exposed pad is connected to GND. Stupid. I removed 2 chips, cut away the exposed pad and replaced them. Now it works! Thanks for all the effort.

    @Valerie, I definitely can confirm this!