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LM3560 - 4 or 6 LEDS

Michael Gilge
Expert 1720 points
Other Parts Discussed in Thread: LM3560

Hi,

I was wondering whether the 1.5A per output capability of the LM3560 could be used to drive 2-3 LEDs in parallel, splitting the current between them?

Alternatively, would the LM3560 support 2 or even 3 LEDs (e.g. Vf=1.7V to 2.4V) in series per output?

We are using the Osram SFH4557 NIR LEDs and are looking for a strobe driver.

Thanks!

Michael

  • 0
    TI__Intellectual 1040 points

    Yes, you can drive any number of LEDs in parallel, however there are only 2 current sources (1A total each) so if you drive more than a single LED per current source then you will need ballast resistors in series with each LED to ensure one of the LEDs didn't run away with the current.  The ballast resistor should be sized as large as possible, but there are some limitations

    1. if too large the power dissipation is excessive

    2.  there is a limit of 5V at VOUT, so the combination of VLED + RBALLAST x ILED + 350mV (current source headroom) should not exceed 5V.

    3.  There is a max current limit of 3.6V, so if VOUT gets too high the boost will require more peak inductor current.  This can limit the minimum VIN.

    For the other question, yes, there can be multiple LEDs in series, but along the same lines as #2 above, the max VOUT is 5V, so the series string of LEDs should not exceed VLED(total) + 350mV.

    Thanks

    Travis

  • 0 in reply to Travis Eichhorn
    Expert 1720 points

    Hi Travis,

    thanks for your quick and  comprehensive answer. I will go with 2 LEDs in series at each of the outputs, in order to avoid the ballast resistors and associated power dissipation.

    The supply is 5V, subtracting 350mV leave me with 2.3V per LED. If the current stays below 800mA the Vf of the LED will below that limit. 

    Do you recommend a large electrolytic capacitor in order to provide the flash energy, for example 5000 uF ?

    Thanks,
    Michael