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Original question:

DAC7760: -0.5 V source for AVSS

LM7705: -0.5 V source for AVSS DAC7760

Vladimir Aparin
Expert 2610 points
Part Number: LM7705
Other Parts Discussed in Thread: DAC7760, , LM27761

In continue to DAC7760: required voltage on AVSS

We want to use DAC in 0-36V output range. So we need negative voltage source as low as possible to -0.5V. Because the output range of DAC has limited to 36V and each negative volt on AVSS lead to decrease maximum VOUT.

But all inductorless invertors working from 1.5V and required mirrored positive voltage http://www.ti.com/power-management/non-isolated-dc-dc-switching-regulator/buck-boost-inverting/charge-pump-inductorless/products.html#

So we should use at least 2 ICs - positive regulator + invertor or invertor + negative regulator. These solutions is not pretty.
At the same time all design and EVMs for DAC7760 provide solution for ±5/±15 V.

How TI recommend to implement 0-36V out? What should use do for -0.5V source?

  • 0
    TI__Guru**** 150830 points
    Hi Vladimir,

    Thanks for splitting this to a new thread and referring to the old post.

    I'm confused as to your need for -0.5V and 0-36V. I think you are mainly looking for a negative voltage, which is at least lower than -0.5V?

    What positive voltages do you have available? About how much current do you need on the negative voltage?
  • 0 in reply to Chris Glaser
    Expert 2610 points
    Hi Chris,

    -0.5V - it's requirement for DAC7760 for providing good performance near the zero voltage output, as described in original post.
    Current on AVSS pin is not fixed in datasheet, but I think, that it should be about some mA, for reference output current - 10 mA.
    We have 36V and 3.3V power rails.
  • 0 in reply to Vladimir Aparin
    TI__Guru**** 150830 points
    Hi Vladimir,

    Unfortunately, -0.5V is below what the LM7705 can do and above what other negative converters can do.

    Looking at the DAC7760 D/S, it seems like AVSS can go down to -18V. So, a -1V or -2V rail should work.

    However, it also looks like the sum of AVSS and AVDD should remain below 36V, so your 36V rail would be limited to 34V with a -2V AVSS (as an example). I assume the 36V is the maximum tolerance of your 4-20 mA current loop?

    I recommend the LM27761, configured for its maximum rated -1.5Vout. If you have a regulated positive voltage in your system (a reference voltage), you could bias down to VFB pin to achieve your -0.5Vout. This would be the same technique used in figure 1 here: www.ti.com/.../slva251.pdf