I am using LM3421 in a buck-boost LED driver design. I was wondering if the timing resistor that is originally connected to SW node to setup the OFF time can be connected to the output of buck-boost stage. This will potentially help me to ease the layout, since the buck-boost output rail is a much quieter node compared to SW node. I think during the OFF time period, the stimulus seen by the timing RC is Vin + Vout + Vd when SW is used, and when buck-boost output is used, the stimulus seen is Vin + Vout. Since Vd is so smaller compared to Vin + Vout, voltage proportionality should still be maintained and quasi constant freq should be still valid. Am I thinking this right? I did an experiment using a prototype board and found no significant difference between these 2 methods. I am not sure if there is any downside of using buck-boost output for timing resistor. Please advise.
Thanks,
Lei
