I am using the LM3409EVAL board and have connected it up to a PWM for dimming control. It works except for low duty cycle.
As you can see at 5.9us ontime ~25% duty cycle i experience no ringing. the yellow/green line is the PWM at the EN pin. the blue/purple is drain of the FET at the cathode of the recirculating diode. At 25% duty cycle up to 100% i experience no ringing. as soon as i drop below 5.9us on time, i begin to experience ringing on the recirculating diode.
What could be the cause of this? my output is running at 300kHz fromt he eval board. The dimming PWM is running at 42kHZ. the rise and fall times and well within the capabilities of this setup as they require on times of ns and i am in the us range.
Is this a result of the dimming frequency being too fast and i should drop it to like 1kHz?
[Edit] Did more testing. Changed the PWM from 42kHz to 1kHz. Ringing seems to be constant under the circumstance of t_off > 18.5us. Is this an occurrence that just is inherit in the controller for t_off > 18.5us? would i be able to just add a cap and resistor filter in parallel with the reciprocating diode as a filter to reduce this effect? It seems to probably be the effect of the inductor and diode flipping as the inductor power drops below the Vf of the diode, so i would see this at a normal operation when i just turn the thing off at 18.5us i am assume and it has nothing to do with the switching PWM at all.