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DAC8775: Using External -15V

Part Number: DAC8775
Other Parts Discussed in Thread: LM5166

Hi team,

Customer is planning to use internal +15V & external -15V in voltage mode in order to reduce power dissipation in voltage mode - using internal -15V would drastically increase power dissipation. However, it is observed device consumes high current from -15V even when negative arm is not enabled. 

  1. Is above expected? According to datasheet, VNEG_IN_x is internally shorted to PBKG ==> is this the cause? Is this case, can customer still use external -15V when negative arm is disabled? As this would connect -15V to device substrate. 
  2. Any other setting or workaround to effectively disable the negative arm? Purpose is to reduce power dissipation. 
  • PY,

    I don't know of anyone that may have tested an internal +15V with an external -15V supply. I don't think that it is anything that we've looked at. Certainly, we've looked at external +15V and -15V supplies and that schematic is shown in the datasheet in Figure 137.

    I'm not sure why using the internal +15V in combination with the external -15V would not work. The PBKG node is generally the bulk node connection of the device and is generally is the most negative node on the device. In that case, the -15V would be the correct node connection. It would not be GND or PVSS.

    So in answer to your questions, I don't know if is expected because we have not tested that configuration and I'm not sure if there is a way to disable the negative arm of the buck-boost converter.

    Just to be sure, have they checked the circuit connections to be sure that it is correctly connected? Do they have a schematic they can share? How much current does it consume?

    Joseph Wu

  • Hi Joseph,

    ~150mA is observed and the test is using EVM. 

  • PY,

    I just duplicated the test with an external VNEG_IN_x with the internal buck-boost for VPOS_IN_x. I see roughly the same thing. The current going into the VNEG_IN_x section is moderately large (about 125mA). If I use external for both supplies, I use about 10mA in the positive supply and about 20mA from the negative supply. 

    I don't know why the VNEG_IN_x draws so much current when it has an external supply and the positive side uses the buck-boost. I don't think this was a tested, designed-for, use case.

    At this point I would not recommend using the buck-boost converter in this way. Just to be sure, have they considered this design?:

    It uses an LM5166 to keep the AVDD/PVDD to 12V, while it uses the buck-boost for all other power supply use cases. It seems like it does rather well to keep the power consumption down.

    Joseph Wu