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TLC59108: Is there a minimum output current required for this device?

Jonathan Baiardo
Prodigy 40 points
Part Number: TLC59108

Hello.

I have been studying the 59108, 59108F, and the 59208 parts.  I saw the following information in the data sheet for the 59108 and 59108F:
Constant Output Current Adjusted Through an External Resistor (10mA to 120mA)


and then in the Table 7.3 this:
IO Output current OUT0 to OUT7 (min) 5 mA (max)120 mA


This leads me to wonder if I have an LED that's <5mA I won't be able to turn it on. Q is that true?

The 59208 data sheet does not indicate a minimum value anywhere, however it does not have the ability to detect open circuits like the 59108.

One reason to understand this is that I am trying to design a controller board that can work with a variety of LEDs that vary from 2mA to 30mA.  It is preferred to not have the end customer be required to add resistors externally in order to use LEDs that fall within the range I specified.

I did not see any detailed information on Rds(on) for currents lower than 50mA.  How can I determine what that might be at lower currents?  (Or, do I even care because perhaps the Rds just gets lower at lower current draw?)

Thank you for your time and help

  • 0
    TI__Genius 12915 points

    Hi Jonathan,

    I am little confused what 'an LED that's <5mA' is. You mean the maximum forward current of LED? The current range you point out is the output constant current. You also can use 8-bit PWM to control the output current. So basically minimum output current should be 5 / 256 mA.

    You can refer Figure 21. IOUTvsVOUT of TLC59108 datasheet if you need to calculate output voltage.

  • 0 in reply to Hardy Wu
    Prodigy 40 points

    HI Hardy,

    I will try to clarify a bit.  In one of the devices in which I am trying to blink LEDs, the manufacturer has this in the spec sheet:
    • LED Voltage Range: 10 - 24VDC
    • Draw on LED Resistor: Max of 4mA per LED.
    I tested this device in the lab and see 2.5mA current at 18VDC.

    That product has a resistor in it that obviously serves as a current limiter.  

    There are similar other products that have higher rated currents:
    e.g. "• Maximum operating current (per LED): 30 mA" and another "LED current @ 24VDC = 12.5mA)


    So in order to be able to interface to all of these, what do I do?  
    Regardless of 59108 or 59208, the first step seems to be to reduce my VLED to something less than 17VDC to meet the Vo(max) in the data sheet.

    as I mentioned, the 59108 seems to have a 5mA minimum current in table 7.3.  My worry is that there's something internal to the 59108 that will PREVENT me from turning on an LED that has a max current rating of less than 5mA.  So if i need to light the LED mentioned above at 2.5mA, can the 59108 do that?  The Iout / Rext graph seems to level off at 5mA, and the Io / Vo graph doesn't really help either.

    Perhaps this isn't the device for me?


    what I need is a device that i can talk to using I2C that has 6 or more outputs and that can do the PWM on its own. (The open circuit detection is also a great feature.)
    There do exist several I2C-to-parallel output chips, but those do not have the PWM controller.  
    IS the 59208F the device to use here?
    Is there is a device from TI that I have not found yet that can do these things?

    I hope that I've clarified enough my problem. Thank you for your help.


  • 0 in reply to Jonathan Baiardo
    TI__Genius 12915 points

    Hi Jonathan,

    Thanks for your explanation. 

    Vo(max) in the data sheet relates the voltage on output pins, which equal to VLED minus voltage drop on LED. To protect IC in case LED shorted in some extreme situation, it is suggested VLED do not exceed Vo(max).

    PWM control could set average output current to 2.5mA. But the maximum output current cannot be lower than 5mA. if your LED could not be stressed by it, 59108 may not suitable for this case.

    Output current PWM are controlled by register 02h~09h. You could find details in TLC59108 datasheet page 24.