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LM22670 adequate for Conversion +5V -> -3.3V, 2A ?

Christian Honisch
Prodigy 60 points
Other Parts Discussed in Thread: LM22670, TPS54521, LM3481

Hello,

I need an inverting switching regulator for the conversion from +5V to -3.3V and a current of 2A. In another application I am using the LM22670 for the conversion from +12V to -5V. Because of this, I want to adopt the design to the new voltages. I think that it should be possible, but I am not an expert on switching regulators. The only issue I found in the app-note (AN-1888) is that the phase margin decreases at input voltages below 6V. Because of this one should increase the input capacitors. Which value will be adequate?

Is there any other important thing I have to consider, when using the circuit for +5V -> -3.3V?

Best Regards,

Christian

  • 0
    TI__Genius 16025 points

    Hi Christian,

    I've attached the modifications you would need to make this application suitably work with LM22670. You will need a 5A buck if you want 2A Iout. I would recommend using 47uF for C2, and 20uF combined C6 & C7. Please see attachment for schematic and recommendations.

    3364.E2E Simple Switcher - Christian Honisch.pdf

    If you want -3.3V@2A output, remember for a conventional inverting buck boost topology, The average switch and inductor currents is the sum of Iin and Iout.

    If you have any further questions, don't hesitate to ask.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 60 points

    Hi Anston,

    Thank you for your reply.

    Another option for me might be to change the input voltage to 12V. Actually, I would prefer to use 5V, but I need a quick solution without building a prototype. If I didn't make a mistake, equation (8) of the datasheet gives I_max=2.25A for V_in=12V. Can you please confirm this?

    However, for long term use I am interested in a solution for +5V -> -3.3V, also in +5V -> -5V (@ 2A). Is there a buck regulator you would recommend for this? I would prefer a chip with a reference design, if there are several possible. By the way: The fact that there is a reference design for the LM22670 for negative voltages was the main reason for my decision to take this chip.

    Best Regards,

    Christian

    Edit: Maybe, 1,6A is also enough, the design is not yet final. Using formula (8) from the app note, I get I_max=1,65A for V_in=+5V. Is this correct?

  • 0 in reply to Christian Honisch
    TI__Genius 16025 points

    Hi Christian,

    We don't have reference designs for +5V to -3.3V. For your first case, 12Vin to -3.3Vout, Imax of 2.25A should be possible without any issue based on the reference design. Take note that the device voltage is actually 15.3V.

    For your second case, if you need low output noise and don't mind using a 1:1 inductor, the LM3481 in Cuk topology is an excellent choice. Here is another part that might work TPS54521. If you want details on the Cuk schematic, we can draw one out for your application. 

    If you have any further questions, don't hesitate to let me know!

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 60 points

    Hi Anston,

    Thank you for your help. I will take the LM22670 for my application. However, can you please confirm (or negate) my result of I_max=1,65A for +5V->-3.3V? That would be great. Then I will mark this topic as answered.

    The LM3481 in Cuk topology might be just what I need. It would be nice if you can provide a schematic to me. If possible, I would prefer parts in the schematic that are available at digikey.

    Thanks in advance,

    Christian

  • 0 in reply to Christian Honisch
    TI__Genius 16025 points

    Hi Christian,

    I can confirm that I_max  = 1.65A would work for +5V->-3.3V. I've attached the Cuk topology based on the LM3481 schematic here with as many parts from Digikey as possible. 5100.E2E Simple Switcher Q2- Christian Honisc.pdf

    Let me know if you have any further questions.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 60 points

    Hi Anston,

    Thank you very much for the schematic. It is exactly what I was looking for. I'm really looking forward to try it out. However, I would be very happy if you or one of your colleagues could review my pcb layout once I have finished it. Can I replace the polymer capacitors by ceramic or low ESR tantalum capacitors? A high long term reliability is important in my application and I think that ceramic and tantalum have a higher lifetime than polymer capacitors have. Please correct me if this is wrong. Can I use the schematic also for +5V -> -5V? Is there any modification beside the gain resistor of the Opamp necessary?

    Maybe one final question concerning the LM22670: Can I use 100nF (0603 or 0402) for the boot-capacitor instead of 10nF?

    Best regards,

    Christian