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LM3642: thermal concern

Kelly Wang
Intellectual 550 points
Part Number: LM3642
Other Parts Discussed in Thread: LM3648, LM2759,

I am designing a led driving circuit for LED Luxeon LXML-PWC2.  LXML-PWC2 forward voltage is 3.1V at current 1A. I am planning to drive it at  700mA, where the led voltage would be 3V.  5V input voltage will be used. Strobe signal will be 30Hz. The datasheet claims that the thermal resistance to ambient is 100degree/W. I would like to know, would thermal be an issue for my application no matter how the layout is?

Thanks,

Kelly

  • 0
    TI__Intellectual 1940 points
    Kelly,
    In your application, the power dissipation during the current on time will be high regardless of your layout, however as you will be pulsing, the average power could be manageable. Additionally, I don't believe this device will work well because I don't believe it allows an easy repeated on/off process. To get this to work with this device, it would require I2C writes before every pulse to re-enable the part. A device like the LM3648 would prove to be a better solution as there is a way to easily get repeated pulses.

    Greg
  • 0 in reply to Greg Lubarsky
    Intellectual 550 points
    Greg,
    Thank you for your reply. LM3648 will be a better solution from the data sheet description. But from the data sheet, I can't see exactly how fast the response of LM3648 is. Will LM2759 have similar repeated pulse on/off performance as LM3648?
    As you know, I am also considering using LM2759 to drive this LED. In my design, I need to drive 2 LEDs alternatively, i.e, when one is on, another is off and their output current can be configured separately. However, both LM2759 and LM3642 have fixed I2C address. Then, I have to use 2 different single LED drivers to drive two LEDs. Current of LEDs will be configured at the start up and no change during the strobe active time.

    Best,
    Kelly