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Original question:

OPT8241: What is role of comparator and XOR?

OPT8241: Current regulator and

Dahwan Park
Prodigy 90 points
Part Number: OPT8241
Other Parts Discussed in Thread: SN74LVC2T45, , TPS2559, CSD17578Q3A, CSD16301Q2

I'll design our laser driver circuit. I plan to design shunt switching with four laser.

Used laser's operating current is ~3.6A and operating voltage is ~2.2V. Therefore, I'll use current regulator to limit max current.

opt8241 model uses TPS2559 as current regulator and it limits current up to 5.6A and uses SN74LVC2T45 as trigger.

I have two questions.

1. Could you recommend adjustable regulator? (limit current value : 14.4A)

2. Additionally, Could I use same power MOSFET model in OPT8241 ? ( CSD16301Q2, CSD17578Q3A)

Thanks.

  • 0
    TI__Intellectual 1680 points

    Hi Park,

    Do you mean to say you are planning to put all the lasers in parallel to the NMOS ? That may not be an optimal solution as the operating points can be different for different lasers resulting in different phase offsets . In the CDK, all 4 lasers are in series and the total string is kept parallel to the NMOS.

    regards

    Nithin

  • 0 in reply to Nithin Jose
    Prodigy 90 points

    I understood your advice, so I'll connect all 4 lasers in series. In this case,I think I can use same current regullator of OPT8241(TPS2559). Because, it's current limit is up to 5.6A. I designed two circuit seperately and I'll insert ILLUM_QUAD_LASER and FPGA_OUT at the same time. Does this circuit operate well?

    Could you advice to me?

    Thanks.

  • 0 in reply to Dahwan Park
    TI__Intellectual 1680 points
    Hi Park,

    How many lasers are there in your circuit ? Have you checked the optical power requirements using the system estimator? Can you specify your use case, spec requirement and power requirement ? With these many lasers, how do you plan to meet eye safety requirement ?

    Regards,
    Nithin
  • 0 in reply to Nithin Jose
    Prodigy 90 points
    Hi Nithin,

    We want 25W and 60 degree beam divergence and okay to high modulation frequency(~60MHz). I used 8 laser that optical power is approximately 3W. Eye safety is not important because we'll test wearing IR protect glasses and coat.

    Thanks,
    Dahwan Park
  • 0 in reply to Dahwan Park
    TI__Intellectual 1680 points

    Hi Park,

    can you explain how you derived at 25W power requirement. Is it electrical or optical power ? What is the depth accuracy you are looking at ? Did you use the system estimator tool to calculate your specs ? 

    The circuit shown above also will have the same issue. Any mis -  match in currents will cause a difference in forward voltages of the lasers. This might eventually cause one string to be on and the other to be off. Since both strings are connected to the same current source, the current limit you are setting will be double of the individual string. This will result in thermal runaway of the lasers.

    Regards

    Nithin

  • 0 in reply to Nithin Jose
    Prodigy 90 points

    Hi Nithin,

    25W power is electrical power, so optical power may be approximately half of that value. It is good as optical power is stronger.

    I don't understand exactly your advice. If current regulator supplies 6A constantly,I thought left circuit and right circuit will get 3A. Because structure of left circuit and

    right circuit is symmetric.If large mis-match occurs for a delicate difference and left circuit will get 2.9A and right circuit will get 3A, I thought both 8 LD will turn on.

    Because LD's threshold current is 0.2A and it's Absolute Maximum Ratio(AMR) is 4.5A. Can you explain more detail about any-mismatch you said?

    Best Regards

    Dahwan Park

  • 0 in reply to Dahwan Park
    TI__Intellectual 1680 points

    Hi Park,

    Consider a simple case of 2 lasers in parallel with approximate forward voltage of 2.2V at the operating current of 3.6 A. You have a current regulator that limits the current to 7.2A. The current regulator output can support upto say 2.5 V.  Now imagine that due to manufacturing tolerances, cut in voltage of laser 1 is 2.1V and that of laser 2 is 2.3V. Laser 1 will turn on first and holds the output voltage of the regulator at 2.1V thus never turning on laser 2. Now the entire 7.2A current flows through laser 1 and leads to burning it. After that the  output voltage will adjust for laser 2 and the entire current will flow through it eventually burning that also. To avoid this you can keep two resistors (one in each string , in series with the lasers ). But that will be an extremely inefficient way as you will waste power.

    That is the reason, we are asking your use case and spec requirements so that we can help you with a better approach.

    Regards

    Nithin