LM3421 LED driver IC.....off time

[treez]

Hello,
The off_time of the LM3421 LED driver depends on the input voltage...the higher the vin, the longer the off_time.
However, in the datasheet, they recommend that when using buckboost topology, the RT resistor is connected to the Anode of the output diode...

LM3421 datasheet
https://www.ti.com/lit/ds/symlink/lm3423.pdf

..eg page 56 of datasheet

Why do they do this?....because this actually cancels the effect of getting longer off_time as vin increases.
Why don't they connect the RT resistor to say a fixed 10V or something?

 

[BradtheRad]

The buckboost output is at inverted polarity from the power supply. That may be the reason they singled it out and recommended different treatment.