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.