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LMV2011: "Precision Method for Laser Diode Emission Control" Help

Tiffany Greyson
Prodigy 80 points
Part Number: LMV2011

TI's Precision Method for Laser Diode Emission Control app note shows a way to use the very small current generated from a photo diode to control the drive strength of a LED/Laser Diode.  I want to build this circuit using the integrated photodiode in the laser module.  My problem is that in a typical laser diode 3 pin package, I have access to the photo diode's anode but not the cathode.  The cathode is tied to the laser diode's anode which is tied to VCC.   Below, pin 2 will be tied to VCC, pin 1 to the PNP like Figure 2 of the app note.

How do I connect pin 3 to the opamp so that I can amplify the small current into a precision voltage? 

  • 0
    TI__Mastermind 23050 points

    Hi Tiffany,

    I believe you can accomplish this with some minor tweaks to the circuit in figure 2 in the app note you've attached (see below).

     

    LED Current Control.TSC

    Please note that the values I've chosen here are arbitrary and you should carefully select these components to satisfy your requirements. In this instance, the photodiode must obviously be able to tolerate the reverse bias voltage of Vcc without excessive leakage current. Functionally, the circuit is very similar to figure two, but now on startup the op amp output will saturate high, forcing the NPN transistor into saturation. This will cause current to flow in the LED, which will produce a current in the photodiode that will flow through R1. The op amp will drive the transistor to shine enough light on the photodiode for the voltage at the inverting terminal to equal the bias voltage applied to the noninverting terminal (1.25V in this case). Thus, the necessary photodiode current is set by Vbias/R1. I hope this helps get you started!