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TLC59108 without Rext

Levente Tamas
Prodigy 110 points
Other Parts Discussed in Thread: TLC59108, TLC59108F

Hi, I am using a TLC59108 in my project, but I have just noticed that there is no Rext on my board. What is the effect having that pin floating?

So far I didn't see any misbehavior, works as expected (except for one output, which probably broke while I was programming the device in RESET state... which should go into the manual, that it is a bad idea also a very hard to detect -no pwm, or small blinks at full brightness and so on-  :D ). 

In my design the outputs have mosfets on them to drive more power, and each pin has to sink about 12mAs.

Thanks,

Levente

  • 0
    TI__Expert 5275 points

    Hi Levente,

     

    Thanks for your question and the feedback on the manual.  As for not having Rext, this is not a problem, but it will set the output current to the lowest setting of ~10mA.  Reference Figure 12 in the Datasheet for the impact of Rext on the Output Current:

     

     

    This might cause an issue for you if your target current is 12mA.   Plus, I think this won't let you adjust the brightness (output current) since the Iref will be at its minimum.

    Also, how are the mosfets being used to drive more power through the LEDs? 

     

    Kind Regards,

    Adam Hoover

    LED Driver Applications Engineer

     

     

  • 0 in reply to Adam Hoover
    Prodigy 110 points

    hi, so I should definitely add some resistor there. I have just realized that it is 6mA, not 12. it was a while ago...

    Do you have any idea what can be wrong with one of the outputs? It broke while the device was programmed in RESET being low, some flickering was going on, and weird things, but once I have fixed the Reset issue, cannot be turned off any more.

    This is how I am using it:

  • 0 in reply to Levente Tamas
    TI__Expert 5275 points

    Hi Levente,

     

    I just realized from your schematic that you are using the 'F' version of the part.  The TLC59108F actually does not have an Rext pin, it just uses the resistors to limit the current.  The 'non-F' version TLC59108 has constant current outputs and uses the Rext to set the current.  

    Now that we've got that squared away, I don't see any reason why the device would have gotten damaged in reset.  You said you were pulling the external reset pin low and tried to program.  When the reset pin is low, "the registers and I2C state machine will be held in their default state".  So all the outputs should be turned off, and since I2C programming shouldn't have an effect since it is disabled.  

    The LEDs are downstream from the FETs correct?  (Nets: STRIP1_B, STRIP2_R, STRIP2_B)  Is it possible one of these were damaged and stuck open?  Can you check to see if the output still pulls 6mA of current when you turn it on (bringing the gate of the FET down to ~6V)?  And then check if turning it off stops the current (brining the gate of the FET to ~12V)?

     

    Kind Regards,

    Adam

  • 0 in reply to Adam Hoover
    Prodigy 110 points

    Hi Adam, good to hear that it has no pin, I have already installed it on the ceiling so I couldn't check. ;)

    So in theory the reset pin should keep it in reset, I suspect that it was sometimes in reset and sometimes it wasn't. What was weird is before I forced the reset pin (which comes from a wifi xbee module's gpio) to high, the whole dimming suddenly started working. Until then (well the module's documentation says that pin is an input, but I don't believe them any more) I was able to program the registers and read them out. I started suspecting something when I read the last two registers, one of them being the Iref and the other one is the error, there I saw weird things. They never assumed their power on defaults, and every read of the error register got different values back. And as I mentioned before all others were working ok. Also changing the outputs to fixed on/off states (not dimming or group dimming) seemed to be working ok. 

    So it might be that it had nothing to do with the reset signal, but it is more than interesting, that by just setting that pin high suddenly fixed everything except for my single bit error. 

    I suspected myself the fet as well, but since luckily I still have 2 more pin header outs right next to these two I am using there currently, so I'll just shift the cable 2 positions down to avoid the bad output all together.

    Thank you very much for your professional help, and I think we can have this as case closed. 

    have a great weekend

    Levente

  • 0 in reply to Levente Tamas
    TI__Expert 5275 points

    Hi Levente,

     

    Happy to hear you were able to find a fix.  Thanks for the additional info you provided on what's happening in this system.  I'm glad I was able to help you!!  If you do come up with any more questions, just let us know!

     

    Best Regards,

    Adam