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TPS7A4001: Regulating a large negative voltage

George Fleming
Prodigy 40 points
Part Number: TPS7A4001
Other Parts Discussed in Thread: LM337

I seek to have a very low noise dual rail supply (+-42.5V/50mA). The power rails will be generated by two isolated 48V to 42.5V DC/DC converters. To clean the positive side, I can simply use the TPS7A4001 as recommended to achieve the 42.5V/mA output. However, I cannot find equivalent performance and voltage rating with the negative LDO selection, but I read that I can connect the positive input of the negative supply to VIN, negative input of supply to GND, then connect the output of the LDO to GND to achieve the -42.5V regulation. I can do this as long as the negative supply is galvanically isolated. Will this work specifically with TPS7A4001? Anything I should watch out for?:

  • 0
    TI__Mastermind 49925 points

    Hi George,

    Few things here.

    First, the LDO will need more headroom to regulate, so the DC/DC cannot have a ±42.5V output if the LDOs need a ±42.5V output. If the DC/DCs can regulate to ~45V this will be fine. Do you need the ±42.5V outputs from the DC/DCs for something else?

    Second, the way you have drawn the schematic is not correct for the TPS7A4001 in general because the device still needs the feedforward cap connected in the same way that it would be when it is connected in a normal positive configuration. Specifically, the feedforward cap needs to be across OUT and FB. Remember that the device is still trying to regulate FB to VREF relative to the voltage at GND. 

    Now, the way you have drawn the schematic (assuming the CFF is changed to be across OUT and FB), the -42.5V node won't have the current drive capability of the LDO because it is only supplied through the feedback resistors. Also, with this configuration you are putting a positive LDO in the ground return path, which is not ideal. 

    Have you considered using the LM337? The noise performance is not as good as the TPS7A4001, but the implementation will have less gotcha's. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 40 points

    Thanks Nick, 

    To clarify, the DCDC output is 48V going into LDO.

    I fixed the resistors and cff. I simulated in TINA, but the results aren't quite what I would expect (regulated to -43V). I may just try and breadboard it to see what happens. 

    If I am not mistaken, the LM337 only is capable of output voltage of up to -37V.

    Regarding the current drive on -42.5V, are you sure that the IN->OUT->LOAD->-42.5V regulation loop won't cause -42.5V to regulate with reference to output (which is tied to reference ground). I would expect in steady state that the voltage at negative side of input DC/DC to be -42.5V with reference to ground, positive side of DC/DC to be ~5.5V, OUT to be 0V (tied to load ground) and GND reference pin of LDO to ride the -42.5V rail.

    I would also expect the ground return current will not pass from OUT to IN of DC/DC as it is isolated to negative side loop (current will always flow out of DCDC 48V rail and return at -42.5V node)

    /resized-image/__size/1920x1920/__key/communityserver-discussions-components-files/196/2311.tina.png

    H    ere is an example online of what I am trying to do (but with different components and voltage levels):
    Negative voltage from a positive LDO - Electrical Engineering Stack Exchange

  • +1 in reply to George Fleming
    TI__Mastermind 49925 points

    Hi George,

    Got it. LM337 has a VIN - VOUT maximum of -37V, but it has a floating-ground topology so for your application the headroom would only be the -5.5V. I was initially under the impression that you had a DC/DC that was regulating to +48V and another regulating to -48V, so if you went this route you would need to get the -48V with a charge pump or something like that. 

    I think understand what you're trying to do now. Do you need to keep the supplies isolated? I'm looking at the example and the way it was done the supplies are not isolated because they used the positive-side ground as a current path and reference point to get the negative voltage. It looks like you are trying to do it the same way in your Tina schematic. I'm not sure you can keep the supplies isolated and get a negative voltage like this. If you don't need the supplies isolated this should work. 

    Regards,

    Nick