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LT3743 used for multiple Lasers

I have used the LT3743 in the past where we drove lasers with large currents ( 10-15A).  For this new project, i have 3 lasers with much smaller currents.  This part might not be the ideal part and I am open to another suggestion, but I'm somewhat familiar with this part and I don't mind using it on this project.  It would be ideal if I could duplicate the circuit 3 times and use any of the lasers on any of the circuits.  However, if this isn't manageable, I am willing to have some jumpers that I can configure to set up a circuit for a particular laser.  My 3 different lasers are:

Blue 160mA max with a typical Vf of 5.2V and 6.5V max

Green 145mA max with a typical Vf of 6.9V and 8V max

Red: 200mA max with typical Vf of 2.7V and 3V max.

I am thinking that I can use a 0.15 ohm sense resistor which limits the max current to .333 A and that should work for all 3 lasers.  When I try and calculate the inductor using the equation in the LT3743 datasheet, I get a wide variety of values that seem really high.  I think that one value could probably work with all three lasers but I'm not sure how different inductor values will affect the operation.  I don't really care about efficiency and since my currents are small, I don't care about heat.  I chose 1Mhz for the switching frequency and my VIN is currently 24V.  However, I do have a swticher on the board that is generating a lower voltage of 8-9V that I could also use as the VIN for this part which might help.   So I have a question about choosing an inductor that might work with the three different lasers.  Another question I have is that since I'm not doing any fancy current switching between a high value and a lower value is it okay to not use the switched capacitor output sections on the PWMGH and PWMGL pins?  Do I just need a single FET and use the PWMGL pin like the sample application on page 22 of the datasheet?  Other parameters are that I'm not using the thermistor input so I have that pulled high.  I don't need to switch between CTRL_L and CTRL_H.  I plan on using a DAC to send a control voltage to the CTRL_H pin to allow me different current outputs up to my max for the laser that I have installed.  In this instance, it seems that the CTRL_SEL, PWM and Enable pins are all the same logic.  I might be able to tie the PWM high, CTRL_SEL high, and then just use the Enable pin to enable and disable the current output.

Thanks,

Keith

  • Thanks for the post.  In general, controllers like the LT3743 can handle a wide range of current levels compared to monolithic ICs.  That's because the signal levels in controllers are based on the external sense resistor instead of an internal fixed sense resistor.  In other words, you can take a 20A LT3743 design, then increase the inductor and RSENSE by 100 and decrease COUT by 100 and the controller won't know the difference in a sense between the 20A and 200mA designs.  I just tried that experiment in LTspice using the LT3743 test fixture.  I accessed the LT3743 test fixture as I show in the image below by adding the LT3743 component to a blank schematic and the clicking on the "Open this macromodel's text fixture" button.  I think the LT3743 test fixture shows as well that the LT3743 works o.k. without the capacitor switching circuitry that you mention in your post.LT3743 test fix_1.png