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DAC adjusted current source circuit

Sainer Siagian
Intellectual 370 points
Other Parts Discussed in Thread: LMZ21700

Hi. I am testing an adjustable constant current source circuit as described in attached file. I am testing it now with LMZ21700. I know that LMZ21700 works in DAC adjusted Vout, but does it also work in DAC adjusted Iout circuit?. Are there any concerns or requirements when using the circuit?. In case LMZ21700 doesn't work, can you suggest any ICs that can work with the circuit?. My system requirement as following.

1. Inductor integrated DC/DC converter.

2. DAC's Slew rate 1.2V/us. DAC Full Scale =2.5V.

3. Time to settled　Vout = 2ms(Max)

4. Load resistance = 30 to 40 Ohms

5. Iout range = 60 to 120 mA.

  • 0
    TI__Guru 57555 points

     

    I do not see any problem with using the LMZ21700 in this application.  Of course this design will have to be checked on the bench before committing to a final design.  Regarding the slew rate and settling time, this will have to be checked on the prototype on the bench.  Keep in mind that the maximum output for the LMZ21700 is 6V.  Also, be sure that the common mode range of your op-amp is compatible with your load voltage range.

  • 0 in reply to Frank De Stasi
    Intellectual 370 points

    Hi Frank, thanks for your reply.

    Are there any limit or requirements to mantain the loop stability of the circuit?. In case of DAC adjusted Voltage control, according to  following link

    https://e2e.ti.com/support/power_management/simple_switcher/f/858/t/410991

    I have to keep Rfbb < 400kOhm, do I also have to do that on this application?. (In my attached file it refers to R2?)
     
    FYI, We test the circuit in the past with ALTERA's EP5358LUI, but we found that in some probability (around 1/64), the circuit was ringing or stuck at 0 V output. If we replaced it with another one it fixed. It seemed like there were variation of the compensation network circuit inside the IC, that made them sometimes ringing on one, but not on another. On the datasheet of EP5358LUI it is also written that we have to choose 237 kOhm for the upper resistor of VFB to mantain loop Gain.(VID mode)
    In case of LMZ21700 are there the same requirements?.

  • 0 in reply to Sainer Siagian
    TI__Guru 57555 points

     

    Stability is always a concern.  I would suggest using the SPICE mode for the LMZ21700 and test you design with both simulation and a prototype.

  • 0 in reply to Frank De Stasi
    Intellectual 370 points

    Hi, Frank.

     

    Thanks for the reply. I have 2 more questions for you.

    1. What is the minimum value for the CSS?. On page 13 of datasheet it is written that 1000 pF, but it is too slow for me. Can I set the Tss around 235 us (CSS=470pF) ?

    2. Connecting VOS pin.

        On page 25 it is written that "Use a thin and short trace from the VOUT terminal of the output capacitor to the VOS pin". Is it a reason why it has to connect to the output capacitor?.  In case of  DAC adjusted Vout, I want to sense the voltage  of my Load (not the output Capacitor), so I want to connect the VOS pin  to one pin of my Load. Is it a problem to do that?.

  • 0 in reply to Sainer Siagian
    TI__Guru 57555 points

    Let me pass this to the apps engineer for the LMZ21700.

     

  • 0 in reply to Sainer Siagian
    TI__Mastermind 18940 points
    Hi Sainer,

    Here are my comments.

    1. The minimum value for Css for "monotonic" startup is 1000pF, across the temperature range of the device. You can try using a lower value of Css, but the vout ramp up may not be as clean and you will see larger inrush current.

    2. The Vos connection to the output capacitor is part of the control loop. This connection should be low noise, which is why the datasheet suggests a short and thin trace directly to the output capacitor.
    Do you expect a large voltage drop between your load and the output cap?
    I would still suggest connecting Vos directly to the output capacitor even if there is some drop to your load.

    Cheers,
    Denislav
  • 0 in reply to Denislav Petkov
    Intellectual 370 points

    Thanks Frank.

    Thanks Denislav, I will do what you suggested.

  • 0 in reply to Denislav Petkov
    Intellectual 370 points

    Hi Denislav,

    Hi Frank,

    We were working with DAC adjusted Voltage Source with LMZ21700 and it seemed work very well.

    We are working with DAC adjusted Current Source right now using Zero Drift OPAMP but it seems like Chopper-stabilized OPAMPs need compensation capacitor on its feedback to make it stable. We already tried with OPA2188AIDK from TI and AD8629ARMZ from Analog Devices.Do you have any suggested simpler OPAMP to make a balanced loop?. Because of the limited PCB's occupation area, we required the same size (or smaller) and 2in1 OPAMPs. Sending me schematics to following email would be really appreciated.

    sainer.s@gmail.com

    Notes: The only change to schematic attached on my first post is Rcs and Rf become 100 mOhm, 75 kOhm respectively.

    . 

  • 0 in reply to Frank De Stasi
    Intellectual 370 points

    Hi Denislav,
    Hi Frank,

    We were working with DAC adjusted Voltage Source with LMZ21700 and it seemed work very well.

    We are working with DAC adjusted Current Source right now using Zero Drift OPAMP but it seems like Chopper-stabilized OPAMPs need compensation capacitor on its feedback to make it stable. We already tried with OPA2188AIDK from TI and AD8629ARMZ from Analog Devices. Do you have any suggested simpler OPAMP to make a balanced loop?. Because of the limited PCB's occupation area, we required the same size (or smaller) and 2in1 OPAMPs. Sending me schematics to following email would be really appreciated.

    sainer.s@gmail.com

    Notes: The only change to schematic attached on my first post is Rcs and Rf become 100 mOhm, 75 kOhm respectively.

  • 0 in reply to Sainer Siagian
    TI__Genius 16025 points
    Your question is better suited for the OP AMP groups on E2E. There are considerations specific to your application that might not be known to us and we are more than happy to help with any power related questions you may have.

    Thanks,
    Anston
  • 0 in reply to Anston Lobo
    Intellectual 370 points
    Hi, Anston. Thanks for your reply.
    I have a question for you. Do you have a frequency response Spice model for the LMZ21700?. It seems like the model provided on TI's website is just Transient Model.
  • 0 in reply to Sainer Siagian
    TI__Genius 16025 points
    I put this request through to out modelling group. You can model frequency response on Webench. Have you tried it?

    Thanks,
    Anston
  • 0 in reply to Anston Lobo
    Intellectual 370 points

    What I meant as frequency response model is model for simulating the Gain/ Phase (AC Sim ) of the LMZ21700. My purpose is to check the stability of the Loop when the IC combined with Op Amp. On webpage of the IC, there is only PSpice Transient model available.

    Btw, I didn't see Webench can do the Gain/ Phase simulation.

  • 0 in reply to Sainer Siagian
    TI__Genius 16025 points
    I checked with the model team and they don't have an average model to calculate the Gain and Phase through simulation. There are no plans to make one at the moment.

    Thanks,
    Anston