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LM3409 - stroboscope

Christian Wattrodt
Prodigy 10 points
Other Parts Discussed in Thread: LM3409

Hello,

I am looking for a LED driver with the following properties:
- input voltage 22..26VDC
- maximum required LED voltage 18V
- LED current adjustable from 1A to 5A
- The LED must operate in flash mode, frequency: 0 to 30 Hz (stroboscope)
- The drive time must be 20-500 us.
- It is important that the LED current is present within 5 us,  even if there was no flash for several hours.

I do some tests with the LM3409 demoboard.
I use the EN-Input for controlling the flashlight. There is a delay of 70us until the LED-current is present.
This happens when the operating frequency is less then 50Hz. At higher frequencys the circuit works perfect.
I think the LM3409 goes into a shutdown mode.
Is there a possibility to modify the circuit?

Thanks in advance
Chris

  • 0
    Prodigy 180 points

    Hello -- I'm using the LM3409 in a design now. You should look at the "parallel FET" PWM dimming option. They only list 20KHz (50uS) dimming with the enable pin, but up to 100KHz (10uS) possible with the external FET dimming option. I may prototype it and see how well it does for strobe.

    I'm looking to modify my board so that it can be a both continuous and strobe -- I need to figure out how I can multiply the current by 10 when in strobe mode. The user could tie a control wire high or low in order to set it into either mode.

    Not sure how to handle the different current settings.. I'm thinking a separate sense resistor could be tied in parallel via the mosfet switch when the user selects it. tried prototyping this a bit but it didnt work out as expected... bummer.

    Good luck

  • 0
    TI__Mastermind 25500 points

    The LM3409 will go into a low power shutdown mode some time after EN is pulled low. This time depends on the cap value connected to VCC as this mode is entered when VCC falls below its UVLO threshold. So the only way to prevent a delay is to somehow keep VCC up, which in some applications may be difficult, particularly since VCC is negative referenced to VIN. For short periods of time a much larger VCC cap could do the trick. For longer periods of time you would either need to somehow power VCC externally or provide a wake up pulse every so often (before VCC falls to the UVLO threshold) to the EN pin (using a micro or some other means).

  • 0 in reply to Clinton Jensen
    Prodigy 10 points

    I had the same problem in my application. The actual data sheet states that  "While the EN pin is in a logic low state
    the support circuitry (driver, bandgap, VCC regulator) remains active in order to minimize the time needed to turn
    the LED array back on when the EN pin sees a logic high (above 1.74V)." The data sheet never says about a certain time after which the driver shuts downd VCC.  I think this is a wrong information that may cause misconduct and should therefore be corrected.

  • 0 in reply to Mario Rossi1
    TI__Expert 3800 points

    Do we have a recommended solution to fix this problem? We are trying to work our way around this problem.


    Thanks and Regards,
    Subhash

  • 0 in reply to Subhash Chandra Sadhu
    TI__Mastermind 25500 points
    Yes. In the low power shutdown section it mentions that if EN is pulled low long enough for VCC to discharge below its UVLO threshold the device will shut down. If you use the UVLO pin of the device rather than the EN pin you will get the same dimming performance without the delay. EN pin dimming is great for high frequency dimming, but the UVLO pin works just as well at high frequency and obviously much better at low frequency as it will never put the device into low power shutdown mode and VCC will remain charged for a fast response time.