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DAC8775: 8ch Power dissipation at full tracking mode

Shota Mago
Expert 6580 points
Part Number: DAC8775
Other Parts Discussed in Thread: LM5166

Hi team,

My customer will use 2pcs DAC8775 for their product. So the product has 8ch 4-20mA output.

They would like to operate with full tracking mode to minimize power dissipation at device.

As a result of evaluation, about 3W(8ch total) power dissipation was observed under condition that PVDD=26.4V, Iout=20.8mA, Rload =600ohm.

[Questions]

  • Could you please check register setting from this TI internal link whether the settings are correct to operate with full tracking mode?
    Please look through "Register Setting" sheet.
  • The customer summarized 2pcs DAC8775 power dissipation at each Iout/Rload condition in "Power Dissipation Calculation" sheet.
    Would it be possible to check if these power dissipation is reasonable or not?

Best regards,

Shota Mago

  • 0
    TI__Guru 52663 points

    Hi Mago-san,

    Joe will review this when he is back in the office on Monday.  Thanks!

    Paul

  • 0
    TI__Guru* 92715 points

    Shota-san,


    I've used a DAC8775 to set up the same test. I get less of a current consumption than your customer gets. From my measurements, I get values that are about 0.4W lower than the customer across the range. For my measurement, I get about 2.57W for high IOUT=20.8mA, Rload=600Ω.

    Note that my supply is a little lower than your customers. I used a supply of 25.7V which is the maximum for my supply, instead of the customer's supply of 26.4V. This shouldn't be much of a change because the buck boost should have a very similar power dissipation.

    The calculation looks like this:

    And I've attached the excel spreadsheet to the post as well.

    1401.Book1.xlsx


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Expert 6580 points

    Hi Joe-san,

    Thank you so much for doing same test using DAC8775.

    It seems that the customer's setting works properly with full tracking mode but efficiency is not as good as your evaluation.

    I will suggest to use 12V PVDD to maximize efficiency of internal buck-boost.

    Do you have any comment to improve efficiency or is there any wrong setting for register?

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Guru* 92715 points

    Shota-san,

    I'm not sure why the customer's efficiency is lower than what I'm measuring. I'm just using the standard EVM to make the measurements.

    Can you forward me the settings for their entire register map and a copy of their schematic?

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Expert 6580 points

    Hi Joe-san,

    You can check register map from previous internal link but just in case, I put internal link again here.

    I don't have schematic now. Once I get it, I will share it via offline of internal link.

    Best regards,

    Shota Mago

  • 0 in reply to Joseph Wu
    TI__Expert 6580 points

    Hi Joe-san,

    I got schematic. Please check register map and schematics from this TI internal link.

    For VPOS efficiency like Figure89, is following calculation correct regarding customer's result under 600Ω Rload condition?

    Input Power of Buck-Boost for 8ch: PVDD*IVDD=24.6* 193mA=5.1W

    Output Power of Buck-Boost for 8ch: VPOS*Iout*8ch=15.48V*20.8mA*8ch=2.57W

    VPOS Efficiency=2.57/5.1=50.5%

    Please note that VPOS value is approximate such 600Ω*20.8mA+3V=15.48V

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Guru* 92715 points

    Shota-san,


    I took a quick look at the schematic, but it didn't show me anything that might cause extra current to be drawn from the supply.

    I did note that there was one part of the register settings that was unusual. The configuration for the buck-boost should be at 0x07 instead of 0x06 as they had shown below.

    Also, with the full tracking mode and a current output, I think that the write of 0x0241 will just ignore the PCLMP setting. If they read the register back it would come back 0x0001. Just in case, have them read the register back directly and also measure the VPOS_IN_x and VNEG_IN_x voltages also.

    Your calculation for efficiency looks correct. Efficiency is a measure of output power divided by input power.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Expert 6580 points

    Hi Joseph-san,

    Thank you for your comment.

    I confirmed the 0x06 address it just typo by customer.

    They measured actual VPOS_IN_x and its voltage was about 3V above from each Iout pin regardless of load. VNEG_IN_x is connected to GND.

    So full tracking mode itself looks work correctly.

    I'm asking the customer to try 12V PVDD using LM5166 to maximize efficiency of buck-boost.

    Again, I appreciate every your support as always.

    Best regards,

    Shota Mago