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ONET4201LD laser driver current more than calculated

Sarjak Shah
Prodigy 10 points
Other Parts Discussed in Thread: ONET4201LD

I am using the laser driver ONET4201LD to drive a laser (QL85F6SA). The maximum current rating of the laser is 35mA, for which I decided the current flown to be around 20mA. As per the equations given in the datasheet of the laser driver, (equation 13 specifically) I got the Rmodset as 13.25kohm. Keeping a safety margin, I put the resistance on MODSET pin as 15.81 kohms using a parallel resistance network with fixed resistors and a digital potentiometer. I connected the laser cathode to MOD+ and anode to Vcc=3.3V. On the MOD- pin I connected a resistor of 20 ohms and other end of resistor to Vcc=3.3V. Now when I measured voltage across the resistor terminals, the voltage drop is near 0.8V which gives 41mA flowing through the resistor (20 ohms), almost double of what I calculated. Assuming that same current would flow from the laser also, the laser burns which actually happened. Could someone please tell me if there is a problem with the laser driver or am I making some mistake which I am not able to identify and fix. TIA.

  • 0
    TI__Expert 8915 points

    Sarjak,

    The setup does not make sense to me. Are you trying to use the modulation current to bias the laser? The modulation current is the high speed AC current. From your description you are not providing a DC bias to the laser. A DC bias higher than the threshold current of the laser is required in order for the laser to operate correctly. You then apply the AC modulation current. Are you following the schematic in Figure 24 of the datasheet?

    I suggest you apply a DC bias current through a bias-T as shown in Figure 24 and slowly ramp up the DC current first using a potentiometer on the IBMAX pin and

    monitor the average optical power from the laser. Further information on different output drive options are in the attached file.

    Regards,

    Alex.

     ONET4201LD-4211LD Output Drive.pdf