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LM2759: LM2759 driving single 3V led at strobe frequency of 30Hz

Kelly Wang
Intellectual 550 points
Part Number: LM2759

I am designing a led driving circuit for LED Luxeon LXML-PWC2.  LXML-PWC2 forward voltage is 3.1V at current 1A. 5V input voltage will be used. Strobe signal will be 30Hz. In this case, I would like to know how the driver behave at 30Hz strobe, like the output response, output ripple, output stability, etc. If possible, waveforms from demo board driving similar flash LED would help me on selecting LM2759 as a driver.

Thanks in advance.

Kelly

  • 0
    TI__Intellectual 1940 points

    Kelly,

    I am looking for an EVM to test out your application, however I do have a few comments ahead of time.

    • At 5V in, the device will be in pass mode so there should not be any concerns regarding output ripple and stability.
    • There is about a 2.5ms delay between the strobe pin going high and the current being up and regulating (turn on delays and current ramping time), so this timing must be accounted for in your 30Hz duty cycle calculation.

    Once I get the EVM in hand I will forward along the scope photos showing operation.

    Greg

  • 0 in reply to Greg Lubarsky
    Intellectual 550 points

    Greg,

    Thank you for your reply and your effort to get some waveform of performance.

    Good to know that in pass mode there will not be conerns for ripple and stability.  2.5ms delay is acceptable for our application so far. 

    Thanks,

    Kelly

  • 0 in reply to Greg Lubarsky
    Intellectual 550 points

    Greg,

    I found that the minimum flash duration is 60ms. Does this mean that if the strobe signal is 30Hz, the LED will be always on? If so, would you like to recommend on LED driver with capability  that can be turned on/off 30 times per second with driving capacity to 1A or above using strobe as the control signal?

    Thanks,

    Kelly

  • 0 in reply to Kelly Wang
    TI__Intellectual 1940 points
    Kelly,
    The 60ms is the timeout duration. This is in place for when the I2C is used to control the flash duration, or to protect the LED in the event the external strobe trigger gets hung up. In your case, the timing will be controlled via the strobe/flash pin, so the timeout should not come into play so long as the pin toggles low before the internal timer lapses.

    Greg