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Buck-Boost converter as current booster

BGX561
Intellectual 610 points
Other Parts Discussed in Thread: TPS40211, INA169, OPA564, LM5122

TI,

I'm currently investigating if it is possible to use a Buck, Boost, or Buck-Boost converter as a Current Booster / Buffer stage for an op amp and if there are any examples of this being done.

Its already possible to arrange the converter as a current source by sensing the current and supplying it to the feedback pin. So I would imagine that this should be achievable by creating some relationship between an outer control loop of the main op-amp and the internal control loop of the converter. 

What are your thoughts on this?

Thanks

  • 0
    TI__Mastermind 21720 points
    Hello BGX561,

    Thanks for reaching out with your question.

    It is possible to set up a converter as a constant current source. Please take a look at some of these designs that us the TPS40211 as constant current source.
    http://www.ti.com/tool/PMP8943
    http://www.ti.com/tool/PMP9010

    Please let me know if you have any questions.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Intellectual 610 points
    ,

    I appreciate the quick reply. My understanding is that these two reference designs use a converter that operates in a Current Control Mode. Unfortunately, this current is far too low for my intended application to be comparable. Most of TI's high current solutions >10A are Voltage Control Mode.

    It also appears that the PMP9010 uses the INA169 current sense to provide current feedback to the converter which would not have sufficient bandwidth.

    My current solution is based on an OPA564 in a Howland Current Pump configuration with a modulated input, but we are looking to boost the output substantially higher than the 1.5A the converter is currently able to achieve up to 20A . To achieve the high current, without sacrificing too much bandwidth and to keep the features of the OPA564, without adding 10x more OPA564's as slaves, it seems more practical to instead buffer the OPA564's output into some kind of current amplifier or buffer supplied by a high current buck converter.

    Thanks
  • 0 in reply to BGX561
    TI__Mastermind 21720 points
    BGX561,

    Most of the controllers can be configured to regulate the output current or the output voltage. The two reference designs referenced above use the TPS40211 which can regulate voltage or current.

    What is the application and what is re required bandwidth? This information will help me point you in the right direction.

    Thanks

    Garrett
  • 0 in reply to Garrett Roecker
    Intellectual 610 points

    Garrett,

    We’re building a general purpose constant current source. The goal is to allow the operation at +/- 10A in a bipolar configuration (GND referenced) or +20A in a unipolar configuration (V- referenced).

    When considering the voltage at these currents, it is ok for the voltage range to roll off at high current. So for example, we might want +20 V output at 1.5A , but only +3V at 20A.

    If we’re still talking about power op amps, the goal would be to adjust the power rails of the op amp to minimize the voltage drop across the amplifier and thus minimize the power dissipation.

    As far as bandwidth, we would want full power at 1MHz voltage-to-current controlled modulation, with a variable current range setting.

    My understanding is that using a buck converter and a current sense amp would be too slow for this directly- maybe up to 100kHz modulation maybe. Using the power op amp in an Howland config seems much more suited to drive this kind of general purpose load up to 1MHz with a current buffer. Maybe with some complimentary power mosfets or bjt’s

  • 0 in reply to BGX561
    TI__Mastermind 21720 points
    Hi,

    I don't think it will be possible to modulate the output voltage at 1MHz. TO achieve this the loop would need to be very very fast which would mean the switching frequency of the design would need to be in the 10MHz and up range.

    I think a better solution would be to implement envelop tracking and change the rail voltages of the supplies. This way the output voltage can move relatively slowly and help to minimize the power the losses of the power amplifier. If you are looking for a boost power supply to implement the LM5122 might be a good choice for higher power designs.

    Thanks,

    Garrett