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LM2759 LED current too low

Michael Sushchikh
Prodigy 10 points
Other Parts Discussed in Thread: LM2759

I have placed a LM2759 on a PCB according to the Typical Application Circuit. I am strobing it using the strobe signal for 1000usec. When the flash current is set to the codes 0,1,2 and 3 the actual LED current increases accordingly. However when the flash current code is set to higher values, all the way to 0F, the actual LED current does not increase anymore. The forward voltage of the LED at 1A is 2.6V and Vin=5V.

 

  • 0
    TI__Intellectual 1940 points
    Hello,
    Without seeing a current waveform I will have to guess what is going on. To me it sounds like you are trying to drive an IR LED at a pulse that is too fast for the LM2759 to support at high currents. At light currents the device only has to ramp through a few codes to reach the target. At the higher codes, it takes longer as there are more steps to ramp through. If you look at figure 12 of the datasheet you will see that it takes about 3ms for the current to reach the 1A level. If you pulse width is only 1ms wide, you will never get to your target. If this does not sound accurate based on your system, please let me know and provide scope photos showing the strobe pulse and current ramp waveform. Thank you.

    Greg