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I changed value of resistor R318 from 10 kΩ to 1 kΩ, and that alone significantly improved off-time. From 200 μs to around 25 μs. That is much better. So I looked into datasheet again. I found t...
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#1: Initial revision
I changed value of resistor R318 from 10 kΩ to 1 kΩ, and that alone significantly improved off-time. From 200 μs to around 25 μs. That is much better.  So I looked into datasheet again. I found the answer on page 5, figure 8:  Test circuit:  It's surprising for me, that load resistance has such significant influence on the off-time.