LM3409HVEVAL: AN-1953 LM3409HV Evaluation Board - Iled rise time is too slow & frequency shift with pwm ducty cycle

Martin Hermann

Part Number: LM3409HVEVAL
Other Parts Discussed in Thread: LM3409HV

Hello,

we have two problems with the AN-1953 LM3409HV Evaluation Board.

1. Figure 8 shows 200ns and Figure 9 shows 10ns Iled rise time.
https://www.ti.com/lit/ug/snva390d/snva390d.pdf?ts=1693927568499&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FLM3409HVEVAL
We only have >1us rise time with the demobaord and shunt fet dimming. We teste several of the jumper options without the resulst of Fig 8+9.

2. The switching frequency seem to shift with the dutycycle of the external 20kHz - 100kHz pwm.
Is that correct? Why?

Best regards

10 months ago

0 Xiaoguang1 10 months ago

TI__Genius 14920 points

MH,

1.Have you followed configuration as mentioned in Method #2 - short PIN2 & PIN3 of J1? MOSEFT normal switching time within ~ns or tens of ns.

If yes, then pls check below questions:

I wonder if you have done any change to the board? Do you have any tested waveform that can be shared? (Like Q3 vgs, LED/inductor current, SW voltage), Is the buck circuit working normally when shunt dimming starts control?

2. Yes. When using external parallel FET dimming, a situation can arise where maximum off-time occurs due to a shorted output. To mitigate this situation, a secondary voltage should be used. Please refer to 7 Shunt FET Circuit Modification for details.