Optocoupler for feedback in LNK306 - help!

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Hi, We are trying to use the LNK306 in a low-side buck, constant-current LED configuration. Everything seems to work fine at room temperature, but when we try to test at lower temperatures, the circuit goes into the .8 sec. restart mode on many of our boards. The only thing we can figure is that we have optocouplers with different current transfer ratios. We are using the Sharp PC817 which comes in a variety of current transfer ratios. Will we have to use some kind of current-limiting resistor on the transistor side of the circuit to limit the current at the FB pin so it won't do this? Thanks. Any help is welcome!