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    How to make a 5V negative supply using the LM2576 regulator

    The negative voltage provision is necessary for some circuits. Usually, the best and cheap option is to use a switching voltage converter. Sometimes the power consumption on the negative rail is high, therefore selecting the LM2576 could be a good choice.

    The LM2576 series of regulators are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator, capable of driving 3A load with excellent line and load regulations. In this task, we used it as a positive to negative converter and regulator (buck-boost).

    In this case, the fixed 5V regulator (LM2576-5) has been selected, but you are free to choose the 3.3V, 12V or 15V versions either. The regulator type selection (fixed output) does not make any specific change in the circuit.

    Below is the schematic diagram of the regulator.

    Click image for larger version. 

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    I have used the Altium designer CAD to design a PCB for the circuit. I did not have the footprint of the LM2576, therefore I used the Samacsys Altium plugin to find the available footprint.

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    I built the circuit and tested the output. The voltage is fixed at 5.03V. To check the switching noises, I used the oscilloscope in the AC mode, 50mV division. As the figures imply, the switching noise frequency is around 52KHz and the amplitude is around 210mVp-p.

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    I tried to implement an LC filter to reduce the output noise, but interestingly it did not reduce the noise! Maybe a capacitance multiplier could help but I did not try it, because one advantage of this circuit is a high current delivery (if I don’t mistake, it can be around 400mA) and using the capacitance multiplier can reduce the output current or introduce a voltage drop.

    I’ve attached the Altium files. The board is a single layer PCB. You can modify it with SMD parts or change it based on your own needs.

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    Re: How to make a 5V negative supply using the LM2576 regulator

    Hi,

    is there any question?
    Otherwise it should be moved to the "Show your DIY" section.

    I tried to implement an LC filter to reduce the output noise, but interestingly it did not reduce the noise!
    The problem is the bad GND wiring.

    If you´re not that experienced --> use a two layer PCB with the second layer comletele filled with GND.

    Otherwise optimize GND layout.

    For this high speed switching signals I recommend to add ceramics capacitors, or use low ESR high qulaity electrolytics capacitors designed for SMPS.


    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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    Re: How to make a 5V negative supply using the LM2576 regulator

    Quote Originally Posted by KlausST View Post
    Hi,

    is there any question?
    Otherwise it should be moved to the "Show your DIY" section.


    The problem is the bad GND wiring.

    If you´re not that experienced --> use a two layer PCB with the second layer comletele filled with GND.

    Otherwise optimize GND layout.

    For this high speed switching signals I recommend to add ceramics capacitors, or use low ESR high qulaity electrolytics capacitors designed for SMPS.


    Klaus
    Thank you for the comment

    actually, the switching frequency is correct according to the datasheet information, but I think because of its inherent circuitry, adding a filter did not help. What you suggest does filtering, but I don't think it would be high enough to remove this.



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    Re: How to make a 5V negative supply using the LM2576 regulator

    The shown 200 mV noise is just what you can expect from the simple circuit and layout. The 80 MHz ringing is mostly caused by the switching schottky diode.

    There should be a low inductance cap between LM2576 input and diode anode to close the commutation loop, e.g. 10 uF MLCC. Input and output LC filters can further reduce switching noise.



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    Re: How to make a 5V negative supply using the LM2576 regulator

    Your GND connection is not correct.As you can see, GND Return Path is long and thin.It will create disturbing over Output Voltage
    Double layer is surely an better option.
    Also using Proper Components for SMPS will reduce the appearing problems.(Especially Electrolytic Caps.)
    Click image for larger version. 

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