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Using TLC5947 (or other LED driver) with incandescent bulbs

D. Johnson
Intellectual 700 points
Other Parts Discussed in Thread: TLC5947

I have an application that needs to drive ~ 70 incandescent bulbs, Vbulb = 28V and IBulb is 30mA nominal. Bulbs are backlights for lots of buttons.  The solution being replaced is discrete FETs and many other components.  The biggest issue - As the bulbs start to fail (and they do) the nominal resistance of the filament goes down and the drive current if unchecked goes through the roof.  At which time the bulb usually fails open.

Would this be an appropriate application for the TLC5947 ? (or is this just a bad idea)

Single bulb per output, Vbulb/led = 28Vdc

Ilim set to 30mA

Software would use 0% and 100% settings for PWM

 

Concerns -

Will the part maintain current limit as bulb parameters vary ?

Will an almost shorted bulb damage the TLC5947 ?

Thanks.

  • 0
    TI__Guru 65200 points

    Hello Mr. Johnson,

    To be honest, I do not expect that anyone within Texas Insturments tested if the TLC5947 can work with incandescent bulbs.

    TLC5947 has regulated current sinks therefore the current will stay constant as long as the IC is able to reduce the resistance of OUTx enough to keep the current constant. The only problem you could encounter with varying resistance of the bulb is increased power dissipation inside the IC and if the bulb is almost a short, you will have 28V * 30mA = 934mW in this one OUTx connected to this bulb. I expect that the IC will shut down due to overtemperature.

    So I expect that it will work, but you also might find a condition with the incandescent bulbs which I am not considering due to my lack of knowledge about incandescent bulbs.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 120 points

    Hi

    Brigitte, your coments makes me some questions....

    Following the datasheet, the TLC5947 can driver 30mA with Led voltage up to 30V.

    I uderstand that all led can be ON, with 28V and 30mA (on each output).

    Am I wrong?

     

     

  • 0 in reply to iko ria
    TI__Guru 65200 points

    Hello Iko,

    You are right, all outputs can be on at the same time.

    I expect your question is about power dissipation inside the IC, right?

    If this is the case, if all outputs are on with 28V on each output, the IC will not be able to stand this power. The most power dissipation is not inside the IC, but inside the diodes connected to the outputs. Therefore it is good design practice to minimize the voltage drop on OUTx to limit the power dissipation inside the IC to a minimum.

    Best regards,
    Brigitte