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TPS7A05: Negative Ground Connection

Jan Bock
Intellectual 2650 points
Part Number: TPS7A05
Other Parts Discussed in Thread: TPS723, LM337

Hi Team, 

can you please check the request of my customer: 

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We have a -3.3V supply and need to produce a -2.5V rail out of it. We want to use an LDO to minimize the noise and ripple. The idea is using the TPS7A0508P as seen below:

As you see we don't connect the GND pin off the chip to our ground. Instead we connect it to -3.3V. And instead of connecting the pin IN to a voltage, we connect it to our ground.

* Does this work with TPS7A05?

* What does the customer has to consider to realize it?

* Is there anything what can be improve here. 

Thanks 

Jan

  • 0
    TI__Genius 12511 points

    Hi Jan,

    So in theory, yes you can have a ground reference that is not the actual ground of the circuit. However, their circuit will not function in it's current state. They have input tied to ground, which the LDO sees as +3.3V, and their output tied to ground through Rload, which the LDO sees as +3.3V. This means the LDO will not have anything to regulate. Additionally, they have -3.3V tied to Vout through Cout, which the LDO sees as 0V. This means they have on the output a resistor and a cap in series going from 0V to -3.3V, which will develop brief current, but not force the LDO on. If they instead tied the other terminal of R16 to the -3.3V rail, putting it in parallel with Cout, then it would not have this problem with LDO operation. There is another concern, and that is that the ground would have to source the current as if it was a 3.3V source, it is only roughly 15uA of sourcing current to the load, but it is still not recommended to operate under the assumption that your copper plane can provide this current.

    I went in the lab, and verified that the LDO functions and provides the -2.5V rail, but we still recommend that they utilize the negative LDO's below that are designed for this purpose.

    There exist negative LDO's, it would be much simpler to instead use those so that you are sourcing/sinking current from the sources directly. It is not recommending to source current from a ground plane, and any transients on other portions of the circuit would have an appreciable effect upon the LDO's operation, on top of the current sourcing problem. 

    The Two LDO's below are negative LDO's that would make for a much simpler design.

    The TPS723 is one that they may use,

    The LM337 is another that is a cheaper option, but older with less functionality.

    Regards,

    John