This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TLC5926 - variable R-EXT

Other Parts Discussed in Thread: TLC5926, TLC59116F

In my application I need to drive LED displays with segment currents as low as 0.5mA and up to 20mA. Doing this with the TLC5926 in standard configuration does not seem to be possible. Is it possible to switch the "R-EXT" between two values so that I could have a lower range and a higher range ? I guess the current through R-EXT is used in a current mirror so whatever resistance variation I might do there has an immediate effect on the current through the on-segments. The datasheet does not describe the implementation (but has lots of confusing formulas, e.g. a polynomial where CC0 relates to 2^5 and CC5 relates to 2^0).

  • Hi Siggy,

    Sorry this was overlooked. With 7-segment displays and similar LEDs, usually we encourage people to use devices like the TLC59116F. This device has open drain outputs, so you can put resistors to manually set the current. The drawback is that this device is controlled via I2C, and not SPI.

    If you want, you could still use the TLC5926, but use two different devices, one for the higher current, and one for the lower ones.
  • Hi Harry,

    Thank you for the reply. Maybe I was not clear enough with my description. The TLC5926 has a current range, that can be controlled via the config register, of from 100% down to 8.33% (1/12th of full scale current). As I want to be able to drive the segments of the digits with 20mA (=100%), the minimum current that I can achieve is 20/12mA=1.67mA. This is too bright or it is a too high current for complete darkness, I need to go down to 0.5mA per segment. (This will be an automatic adjustment). But to achieve the 20mA and the 0.5mA, I need to switch the Rext, which works, I have now tried it.


    Regards,

    Siggy

  • Siggy,

    It sounds like you solved your problem.

    you made a comment about the confusing equation (I think you mean equation 7 on page 20). That equation is just changing the binary number to a decimal. Looking at Table 5, CC5 is bit 7 and CC0 is bit 2, so you have the option of choosing a value from 0 to 63. I hope this helps to clarify things.

    Out of curiosity, have you considered turning the output off through the serial communication? That will give you the "complete darkness" you're looking to achieve, and reduces the need to switch between 2 REXT resistors.
  • Harry,

    With "complete darkness" I meant the ambient light, not the displays' light emission. So when the device is in a dark room at night, I want the segments be driven with 0.5mA, while during the day it may be required that the segments are driven with 20mA.  To achieve the 0.5mA I need 3.15kOhm as Rext, for the 20mA I need 938Ohms as Rext. The first Rext yields a range for Iout of 0.5mA to 5.95mA and the second Rext yields a range of 1.68mA to 20mA (and the 1.68mA was too bright).


    The formula could simply have been:

    Iout = 1.26V / Rext * (1 + HC) (1 + D / 64) / 4 * 15 * 3CM-1

    With this all the various equations and gains would not have had to be explained and one can easily work out Rext for the desired current range.

    I wonder why it had to be that the CC0...CC5 bits are in the reverse order. I would have though it would be easy to wire them differently on the chip.

  • Hi all,

    Am going to design for the BICOLOR led red and green for my project. my question is shall i use both red and green in the same IC , because the red and green are having the different current rating and green is having the high rating than red so am using the iref value for red led current value.

    My question is i shall connect both red and green in the same IC or not ??
  • All LEDs will have the same current flowing through them. Just try it out whether one colour shines too much brighter than the other if they have the same current flowing through them. So just put them in series with a resistor and the driver IC is not even needed for this experiment.