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LM3409HVEVAL: LM3409HVEVAL output error

gao li
Prodigy 20 points
Part Number: LM3409HVEVAL
Other Parts Discussed in Thread: LM3409

Keep the original parameters of the LM3409HVEVAL unchanged, switch J1 to Mode 2, use an external 60 kHz PWM signal for triggering, connect a load of 14 LEDs, and supply voltage via a signal generator. The following issues occurred:
 
  1. A huge surge voltage of up to 90 V appeared at the LED+ output, and a large surge also occurred in the PWM input signal.
  2. yellow PWMin50daa3e9fbc62fb82b496982ad24deeb.jpg 
  3. yellow LED+7d8a9d1a90aa3d995c37db8f23cb4693.jpg
  4. Q3-G43fe0248110219e70a9646b19526862f.jpg
  5. The test waveforms are as follows: After changing R1 to 20 Ω and adding a 10 kΩ pull-down resistor from the G-pin of Q3 to GND, the surge at the G-pin of Q3 gradually became smooth, but the waveform at the LED+ output of the load remained unchanged.
  6. Q3-G0f36a2dcf616aec1cab20b62fe9f34a4.jpg
  7. LED+00b56c536278292ff0d399dce937ac1f.jpg
  8. When using a pulse signal directly input to the EN pin for dimming, although the performance at 60 kHz was poor (the LEDs could not be completely turned off), no large surge occurred.
  9. EN wavecf44b36fcd757b12f0c723be0cc816d8.jpg
  10. LED+2429756d8c299103873dec36c4da241d.jpg
  11. To solve this problem, I replaced C8 with a 22 μF capacitor. Upon power-on, the device had no output, the chip was damaged, and an inductor squeal was heard.
 
My Request: How to solve the large surge problem under external PWM driving, and what debugging methods should be adopted?
  • 0
    TI__Mastermind 43390 points

    您好，
    已经收到了您的案例，调查需要些时间，感谢您的耐心等待。

  • 0 回复 Alice
    TI__Mastermind 43390 points

    Hello ，      

    You may keep R1=20ohm to slow down turning off speed of Q3.  Try adding FB or small resistor between VOUT and LED ANODE. Adding small cap on LED anode to GND. 

    Do you mean LM3409 got damaged if changing C8 to 22uF cap? What do you mean an inductor squeal was heard when chip got damaged? Can you capture waveform of  PGATE, Gate of Q3, inductor current and SW waveform? 

    When you test the waveform, pls make sure that prober GND is connected to GND on the board. You may limit bandwidth to 20MhZ to filter high frequency noise coupled to the probe and/or use GND spring during measurement. 

  • 0 回复 Alice
    Prodigy 20 points

    Hello, I have thought carefully about this. Currently, the demo board operates by using a short-circuited load as the switching method. The speed at which the EN pin turns the chip on/off is not as fast as that of Q3. This time difference causes the current in the inductor to have nowhere to dissipate, resulting in an electrical surge. Could you please advise if there is any effective solution to this problem? Alternatively, can Q3 actually be connected between LED- and GND to act as a switch, while the LM3409 does not need to be switched on/off frequently, thereby achieving external PWM dimming?

  • 0 回复 gao li
    TI__Mastermind 43390 points

    Hello,

         Please refer to datasheet or EVM user guide to see difference using En or shunt dimming mode. Both EN and shunt dimming disable switching and LED current drop to 0 when EN/PWM is 0. 

    Shunt dimming is mature solution that used widely in LED driver solution. You need to make sure that they captured the waveform with correct method avoid noise coupling. Do you use ground spring during test and tried the solution suggested in previous suggestions? 

    About "can Q3 actually be connected between LED- and GND to act as a switch", you can't do it in this way. LM3409 works as constant current source with inductor current, if the current path is broken, the inductor current will continuously charge LED and FET until FET damages. 

  • 0 回复 Alice
    Prodigy 20 points
    Thank you. I've resolved the load issue (the previous overshoot was caused by the load voltage exceeding the set voltage). Now I'm going to proceed with the detailed design. Could you please provide the GERBER files for the LM3409HVEVAL as a reference for my design?
  • 0 回复 gao li
    TI__Mastermind 43390 points

    Hello ,

        Usually you can find design files ( schematic & BOM ) for a reference design.

      TIDA-01463 reference design | TI.com

        LM3409HVEVAL comes with a very detailed user guide. The schematic and BOM are in the user guide. So there is no separate folder for design files.

         AN-1953 LM3409HV Evaluation Board (Rev. D)