Applies To These Products

WLD3343

Question

I am using a WLD3343 (with heat sink and fan) with a laser diode (2 W, photodiode and LD isolated, B type). It needs approximately 2.5 A to deliver the 2 W. The problem is that I get less than about 1.7 A from the WLD3343. It is connected using constant power mode, RSENSE: 0.5 Ω and 1 kΩ preset for RLIMIT. VMOD is 0 to 2 V (to adjust power). When VMOD = 2 V, I adjust RLIMIT and the current goes up until 1.7 A and stops there. I am using a 5 V supply and the LD drops 2 V, so I think that the compliance is OK, but eventually I can use more voltage (6 V, still in the safe operation area).

Answer

Your 5 V supply is sufficient to drive your laser at 2.2 A, and you’re not running into the compliance voltage limit of the WLD3343, so let’s investigate the resistor values for RSENSE and RLIM.

The ILDMAX to RSENSE function is nonlinear, and is very steep at high current values. That means that a small change in resistance will have a larger impact on the current limit setting. In order to increase ILDMAX you may need to decrease the value of RSENSE just a bit more, depending on the tolerance of your 0.5 Ω resistor.

Next, the value of the limit current resistor RLIM is determined by the value of RSENSE, and a small change in RSENSE will have a large impact on the limit current.

One solution to your issue is to set RSENSE to 0.4 Ω, which essentially sets ILDMAX to the maximum value, and effectively removes RSENSE from the RLIM equation. Then adjust RLIM to reach the required output current when your modulation function is fully on at 2 V.

Reference the datasheet for the WLD3343 for more details: WLD3343 Datasheet

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