Question
My question has to do with the EXT VSET connector on the WLD3343. How is the applied EXT VSET voltage related to the current supplied to the LD in CC mode? Is it a 1:1 ratio? Also, in CP mode, how is the applied voltage on EXT VSET related to the photodiode current?
Answer
The WLD maximum current range is externally configurable. In Table 3 of either the WLD3343 Datasheet or WLD33ND & WLD33ND_EV Evaluation Board Datasheet, you will see that the value you choose for RSENSE sets the maximum current out of the WLD.
For example, if you want 125 mA max in constant current mode, you use an 8 Ω resistor. If you want 1.25 A, you use a 0.8 Ω resistor. Once you have the maximum range selected, Equation 11 on page 15 gives the transfer function as ILD = VPIN3/(2*RSENSE). If you have it set up for 125 mA max and RSENSE is 8 Ω your transfer function is ILD = VPIN3/16. With a 1.25 A max current, the transfer function is ILD = VPIN3*(0.625). The voltage input range is 2 V to Ground.
In Constant Power Mode, the maximum range is also adjustable. The maximum range again sets the transfer function but now you are using the photodiode current for feedback. You still set up RSENSE (using a slightly different transfer function, 125 mA now uses a 10 Ω resistor and 1.25 A uses a 1 Ω resistor – see Table 3). Then you use RPD to set the photodiode current range (page 12 of the manual). RPD = 1/IPDMAX. Equation 10 on page 15 shows the transfer function: IPD = VPIN2/(2*RPD).
For example, if you have a max photodiode current of 200 µA, you would use RPD=5 kΩ. This makes the transfer function IPD = (0.1E-3)*RPD.