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TLC5947 24-bit LED driver question about setting current

Jonathan Slaughter
Prodigy 70 points
Other Parts Discussed in Thread: TLC5947

(Not sure if this is the appropriate forum but I couldn't find any other place to post this)

The TLC5947 uses a pin to set the current which determines the global led intensity.  I am cascading several of these devices and would like to use one common iref to reduce the resistors needed and have an easy way to set the led intensity across all the ic's(using one pot instead of many). Is it safe to do this? Of course I would have to increase the resistance by a factor of the number of IC's used.  I could add a small resistance to each iref to help with current sharing if necessary. This might be ideal as I would like to limit current as well but I would also like to minimize components as board space is a bit tight.

  • 0
    TI__Mastermind 36110 points

    No this is not possible. Pin R-EXT is basically the output of a voltage reference. Refernce voltages can not be paralleled. There is a resistor from R-EXT to GND needed for each LED driver.

  • 0 in reply to Matthew Day
    Prodigy 70 points

    Are you 100% sure that they cannot be paralleled? I understand that if it is a voltage reference that they cannot but are you sure that the IC uses some type of voltage reference design? What if I were to put some small resistors in series with each device then all to a larger resistor? The small resistors act as current limiting but also allow for the differences in voltage drops to occur between devices.

     

    TLC1->IREF1->R1->R->Gnd

    ...

    TLCn->IREFn->Rn->R->Gnd

     

    (R1 through Rn will have the same magnitude but are physically different while R is the same resistor)

     

    This is actually how I have designed the circuit. R1 through Rn will have the value that gives the maximum current I want and increasing R will allow me to reduce the current for global intensity. I believe this should work as I'm simply lifting the ground w.r.t. to iref. But each iref has it's own resistor also.

     

  • 0 in reply to Jonathan Slaughter
    TI__Mastermind 36110 points

    My mistake.  If you use a separate resistor for each TLC5947 and then connect all these resistors together to a common variable resistor, you can configure your system so that changing the common resistor changes the global constant current value.

  • 0 in reply to Matthew Day
    Prodigy 70 points

    Thanks. I was not clear but I think I've got the information I needed now.

  • 0 in reply to Jonathan Slaughter
    Prodigy 70 points

    One more quick question(which might be somewhat dumb but I just want to be absolutely sure before I send off these boards).

     

    The TLC5947 will work just fine if only some of the outputs are used and the others are not connected at all?  Do I have to set the greyscale code for those to 0 or will everything be fine in any condition?

  • 0 in reply to Jonathan Slaughter
    TI__Mastermind 36110 points

    You can operate the IC with unused outputs left open.  However, if the unused outputs are programmed to sink current but are opened, the IC will draw additional input current as it tries to provide a higher drive current to the unused outputs.  This will not damage the IC, but results in higher power dissipation.  You can eliminate this additional current by programming all unused outputs to sink 0mA of current.