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LM3429 switching frequency drift

Sean Bigley
Genius 17175 points
Other Parts Discussed in Thread: LM3429, TPS92691, TPS92690, LM3424

Team,

I am designing a mid- / high-power work lamp capable of driving up to 15V of LEDs at 1.5A using the LM3429. I'm noticing switching frequency drift as my system temperature stabilizes. I'm working to balance OCL and Irms of my coupled inductor to ensure my radiating fundamental frequency meets my emissions requirements. I want my measured fundamental to be less than 500kHz, but if I push the switching frequency too low, my minimum inductance becomes too large. I've tried connecting the timing resistor to Vsw, as well as connecting it to Vin, and I get drift of about 15-20kHz in both configurations when my nominal frequency is 400-500kHz.

Any suggestions on how to reduce the drift I've been experiencing?

  • 0
    TI__Mastermind 25500 points
    The LM3429 is not a constant frequency part, its switching frequency will depend on the input voltage and output voltage. When an LED string heats up it will drop in voltage and alter the switching frequency. If you need constant frequency you may want to consider the LM3424, TPS92690, or TPS92691. They are all in the same family and are capable of multiple topologies just like the LM3429.