• Resolved

ONET4201LD - RMODSET Calculation

I am going through the calculations for the ONET4201LD and I am running into an issue with the RMODSET calculation. From the datasheet page 20, equation 14 states the following.....

Rmodset = 265V/36.3mA x (1+ 4000ppm/DegC x (25DegC - 60DegC) = 6.3K Ohms.

If you do the above math the number comes nowhere near the 6.3K ohms. One thing I am suspicious about is the 265V. I would have thought that this voltage should be 3.3V. If you plug in 3.3V instead of 265V you get 12.6K Ohms... which is still double the 6.3K Ohms that equation 14 states. Am I missing something here?

Also, in the above equation, 60 is subtracted from 25 which would make the value negative! The ppm/DegC  number in my datasheet is shown as negative. Should the 4000ppm/DegC number above be negative also?

If I am misinterpreting the equation please let me know. Otherwise I need to know the correct way to calculate Rmodset.

Thank you,

Michael Nycz

  • In reply to Michael Nycz:

    Hi Mike,

    thank you for your help. In the following figure you can see my cicuit diagramm. I changed the Ribmax (47k) and the Rmodset (39k), but i still have some problems, a fault occurs (SDOWN=High) and the Voltage Vmonp=0,000V. I couldn´t measure any I/O pin single-point failures (short
    to VCC or GND) and the laser cathode is not grounded.

    Do you have any idea, what else could be wrong?

    Kind regards


    (0R Resistors are not connected / L3 and L4 are ferrit breads)

  • In reply to Justina Neumann:

    Hi Justina,

                One thing that is different between your schematic and mine is that you are operating closed loop mode (PD feeback) and I am operating in open loop mode (NO PD feedback). In my case the PD pin is open and the anode of the PD is tied to 3.3V. It is hard to read from the schematic that you had sent. Is R8 a 0R resistor that is depopulated? If so, this resistor must be set to a value (not open pin) for proper closed loop operation. It would also explain why Vmonp is 0 volts. Look at the datasheet for setting Rapcset. There is a graph (Figure 17) for choosing Rapcset to get a specific Ipd value. The other thing that I noticed are your values for R13 and R14 are 75 Ohms. In a document that I found from TI called "ONET4201LD & ONET4211LD OUTPUT DRIVE CONFIGURATIONS" there is a schematic provided for an AC coupled output like you are using. Their values for the pull up resistors are 40 Ohms. I am using 35 Ohms in my circuit and that seems to work fine. You also have a series resistor in the MOD+ leg of the output. I am using 0 ohms there at the moment. Did you find that the resistor values you are using were needed for the laser? I am using the HFE8192 and I do not have a series resistor there. Other than that, your schematic looks like mine. Let me know if this was helpful. In another reply I will try to attach the TI document for the output configurations.

    Mike Nycz