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# 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

• Mike,

I get the 6.3kohms from EQ 14.

265/0.0363 (1+4000E-6(-35)) = 7300(0.86) = 6278

These equations are just examples and I would not use them specifically to determine the value of the bias and modulation current resistors. I think it is more intuitive to use Figures 15 and 16 to approximate the bias current and modulation current.

The IBMAX, APCSET and MODSET pins have a current range of 0 to 400μA, with 400μA representing full scale for each programming setting. So, for example, on the MODSET pin a current of 400μA out of the pin will result in maximum modulation current and a current of 0μA will result in no modulation current. The programming pins were designed so that a resistor value of 3125Ω will cause the full current of 400μA to be pulled out of the pin. The 3125ohms come from the fact that the voltage on these pins is that of the bandgap which is 1.25V.

The temperature compensation in the part is not very accurate or effective so it is best to be able to adjust the modulation current verses temperature. Mainly an MCU would be used to do this. I have attached a file showing how a current DAC, voltage DAC or PWM signal could be used to do this.

Regards,

Alex.

• In reply to Alex Davidson:

Alex,
Thank you for the response. I understand the discussion above regarding the setting of the resistors. My question was going to be is the relationship linear but I see from the curves that it is not. I am relatively new to using these devices. I have another question....

For a specific laser, what is the criteria for setting the bias and modulation current? I thought that equation 10 in the datasheet used the device parameters to determine the IMOD for the device. Is this not the case? If not, then what criteria should be used in setting a modulation current?

Similarly, For RBIASMAX, I thought that this would be the MAX value that the BIAS would provide. How is the nominal BIAS determined? Does the ONET4201LD sense the device it is driving and set the BIAS with the constraint of BIASMAX set by the resistor?

If there is a document that explains this please point me at it. Or a brief description would be fine as well.

The laser that I am using is HFE4192-581.  The values that I have calculated from the equations are....

IMOD = 9.4mA

TC=-5846 ppm/DegC

RMODSET= 22.4K Ohms (which does not match with the curve you mention in your post).

RMODTC=4105 Ohms

RBIASMAX=92.7 K Ohms

Let me know if you agree with these calculations.

Thank you,
Mike Nycz

• In reply to Michael Nycz:

Mike,

I'm sorry but I did not receive any attachments.

For an AC coupled design, the bias current sets the value of the average optical power and the modulation current sets the value for the laser extinction ratio. The bias current has to be set above the laser threshold current and the bias current - 1/2 of the modulation current has to remain above the laser threshold current. Using the equations are OK for one particular point in time but they provide only a single resistor value. Lasers have a part-to-part variation and variation over temperature so these have to be compensated for by being able to adjust the bias and modulation currents. If the part is used in closed loop then the bias current will automatically be adjusted to maintain a constant photodiode current but the required bias current has to be set with the appropriate value of the APCSET resistor. Also, the MODSET resistor value needs to be adjusted as well to compensate for variation in the extinction ratio. Therefore, it is not really practical to design a circuit with fixed resistor values.

Regards,
Alex.
• In reply to Alex Davidson:

Alex,

So for my specific case, the MAX Ith that my particular device will achieve is 3.7mA. They also say that the absolute MAX Iforward for the device is 12mA. It looks like may of the specs in the laser datasheet are generated assuming an Iforward of 7mA.  So choosing IBIAS at 7mA seems to make sense. Also, based on these values from the datasheet it would make sense that IMOD should be set to about 8mA based on your previous response. Am I assessing this correctly?

Another question:  Is the term IBIASMAX really misleading? Is the resistor value chosen actually setting the IBIAS value? If so, then I would set the RBIASMAX value to 49K which is from equation 20 (RBIASMAX = 343/7mA) and it looks like it corresponds to the value that one could derive from from Figure 15 (it is tough to read accurately from the graph but it looks in the ballpark).

I understand that to accurately produce peak power over the range of operation of the device that a control system is required, I am just trying to set a nominal bias/modulation point to start with. This should be a straight forward thing to do. I am trying to determine the steps to do that. If what I am proposing above is close to that....great! If I am still missing something in the understanding please fill in some of the details. I do appreciate your patience about this. I am just trying to understand the setup.... Not just get a setup that works.

Thank you,

Mike Nycz

• In reply to Michael Nycz:

Mike,

Based on my experience with laser drivers, especially ones with analog control like the ONET4201LD, it is difficult to compute the exact values needed for the control resistors based upon the information from laser datasheets. It is my recommendation to have a flexible design where you can adjust these resistor values, even with a potentiometer if this is all you can use. For debug I always start with a low bias and modulation current (in this case high resistor values) so as not to damage the laser or cause a fault to occur if the fault detection is enabled. I typically ramp up the bias current first then ramp up the modulation current. I have had numerous customers try to test their design and damage the laser because they just keep turning up the currents if they get a low output.

The resistor used on the IBMAX pin has two functions depending if the part is used in open or closed loop. In open loop, the bias current is set with the IBMAX resistor. In closed loop, the IBMAX resistor sets the maximum available bias current before a fault occurs and the bias current is adjusted with the APCSET resistor.

Regards,

Alex.

• In reply to Alex Davidson:

Alex,
Following the correspondence, I wanted to know what the temperatures T0 and T1 represent in equation equation 12 + 13? If our working temperature range is -40-85 degrees, what are the appropriate values in the above equations?
In addition, in equation 18, what the value "630ppm" represents?

Thank you,
Inbal Farbstein.
• In reply to Inbal Farbstein:

Hi Inbal,

Equations (13) is used to calculate the value of the external resistor (R_MODSET) that the user needs to connect between the pin MODSET and GND for the ONET4201LD. This calculation is done for a given modulation current I_MOD [A] at a given operating temperature that the user wants to achieve the modulation current at. So the T0 will be the reference temperature (T0 = 60C) and the T1 will be the operating temperature at which the I_MOD value is desired.

Additionally, TC in the equation refers to the temperature coefficient for the device that is needed to compensate for the temperature variation of the slope efficiency of the Laser Driver, and is calculated as shown in the example with Eq. (15) and (16).

Thanks,

Sri

• In reply to Sriharsha Kota Pavan:

Inbal,

The 630ppm corresponds to the default reference temperature coefficient value for the device if the MODTC pin is left floating.

Thanks,
Sri
• In reply to Michael Nycz:

Hi Michael,

i would like to know, if you found any working solution for your circuit. I use the ONET4201ld and the HFE4291-581 too and need some help with the choosing of the resistors.

Thank you,

Justina

• In reply to Justina Neumann:

Hi Justina,

I am using the ONET4201LD and I have working hardware. I received the following from a TI engineer regarding choosing the Resistor values....

 These equations are just examples and I would not use them specifically to determine the value of the bias and modulation current resistors. I think it is more intuitive to use Figures 15 and 16 to approximate the bias current and modulation current. The IBMAX, APCSET and MODSET pins have a current range of 0 to 400μA, with 400μA representing full scale for each programming setting. So, for example, on the MODSET pin a current of 400μA out of the pin will result in maximum modulation current and a current of 0μA will result in no modulation current. The programming pins were designed so that a resistor value of 3125Ω will cause the full current of 400μA to be pulled out of the pin. The 3125ohms come from the fact that the voltage on these pins is that of the bandgap which is 1.25V. The temperature compensation in the part is not very accurate or effective so it is best to be able to adjust the modulation current verses temperature. Mainly an MCU would be used to do this. I have attached a file showing how a current DAC, voltage DAC or PWM signal could be used to do this.

When he says Figures 15 and 16, he refers to the ones in the datasheet.

The values that I am using are....

RBIASMAX = 49.9K

RMONB = 768 Ohms

RMONP = 200 Ohms

RMODSET = 40.2K

RMODTC=4.7K

Used in conjunction with the HFE4192-58X I am getting an optical power output of between -4 and -5 dBm.

I hope this helps.

Mike Nycz