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onet1151L: Reference Design for laser driver

Michael Nycz
Intellectual 835 points
Part Number: ONET1151L
Other Parts Discussed in Thread: ONET1151P, , ONET8551T

I am using the ONET1151L in a design. In the datasheet the modulated outputs connect to the laser via some ferrite beads however no detail is given on how to specify them. I was wondering if a schematic exists for the eval board which uses both the ONET1151L and ONET1151P that would give some insight on how to define these components. If a tech document exists that handles that then that would be great as well. Let me know if any of the above exists.

Thank you,

Michael Nycz

  • 0
    TI__Intellectual 1715 points

    Hi Michael,

    Thank you for your interest in our ONET products. Yes, we do have schematics with detailed part numbers suggested to design in the ONET1151L and ONET1151P components. Please find attached the screenshot of the schematics for both the devices (in the same figure).

    I'm also attaching a Userguide document for the reference design of 10G SFP+ module from which this schematic has been taken. The reference design, also known as the ONET-10G-EVM (http://www.ti.com/tool/onet-10g-evm)  is a model design for the 10G SFP+ LR modules using TI's 10Gbps optical devices: ONET1151L, ONET1151P and ONET8551T.

    Please go through the design and let us know if you have any further questions on it.

    Thanks you.

    Best Regards,

    Sri 

    6886.10G SFP+ LR Reference Design - TIDU180.pdf

  • 0 in reply to Sriharsha Kota Pavan
    Intellectual 835 points

    Thank you! This is exactly what I am looking for. I did notice that they use mostly 0201 sized devices in this schematic. I would like to substitute 0402 sized devices just because they are easier to deal with (both in assembly and if I need to make changes at a later time). I assume there are no issues with using the larger devices. Is that correct?

    Thank you again,

    Michael Nycz

  • 0 in reply to Michael Nycz
    Intellectual 835 points
    Hello Sri,
    I have another question. I have always (in the past) used these laser drivers in open loop mode. I was looking at the closed loop case and I have a few questions...

    1) In the ONET1151L datasheet it only mentions Ipd with respect to other values (i.e. CR = IBIAS/Ipd, and the register setting show Ipd ranges). My question is, where is Ipd actually set? Or is Ipd set after the BIASC and PDRNG and CR are chosen?

    2) How is the CR set?

    3) Is there an application note that explains the closed loop operation of the laser in more detail?


    Thank you,
    Michael Nycz
  • 0 in reply to Michael Nycz
    TI__Intellectual 1715 points

    Hi Michael,

    Glad it helped. Yes, you may use larger devices as long as they are of similar specification.

    Thanks,

    Sri

  • 0 in reply to Michael Nycz
    TI__Intellectual 1715 points

    Hi Michael,

    Please find my responses below:

    1) For operating the ONET1151L in the closed loop (APC loop), you do not have to set the Photodiode current (Ipd) value. You just need to select the range for it, so that Laser Bias control DAC, BIASC[0...9] is kept close to the center. The PD range can be set using PDRNG[0..1] in Register 0. This PD range will control the resolution of the Bias setting. For instance, when very high bias current value is targeted, the PD range should be set highest. Once this is set, the Ibias is controlled by setting the BIASC register. Use one of the equations given below to calculate the exact Ibias depending on the PDRNG and BIASC selected:

    2) CR cannot be changed digitally. Please use CR=40 for all calculations.

    3) There is no separate app note describing this. We have included the explanation for the APC loop settings and functionality in the Datasheet itself. I hope the above explanation helps you clear it up.

    Please let me know for any further questions.

    Thanks,

    Sri