Current sink

Discussion in 'General Electronics Chat' started by Shagas, Aug 25, 2013.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Hello

    I'm looking at a datasheet of some driver chips that I'm going to be recieving soon.

    http://www.ti.com/lit/ds/symlink/tlc5947.pdf

    24 channel 12 bit PWM drivers .

    It says 30mA constant current SINK .
    Does this mean that I have to connect the anode of the LED to 5v and the cathode to one of the pins ?
    If so , then wouldn't this mean that the PWM values would be reversed
    (4096 - myvalue) since when I output a high on the pin then now current flows and a low on the pin would be current flow. So I would have to reverse my PWM values? :/
    Or does the chip already invert my data

    Also question 2#

    It says 30mA constant current . Does this mean that I can connect my LED bang on without a resistor or do I have to provide at least some dummy resistance to help the constant current driver do its job.

    Info: The constant current is set for all pins by an external resistor.

    Thanks in advance for the help
     
  2. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    No, it means that the maximum current that can flow back through the chip is 30 mA (otherwise you would smoke the IC). The IC will still give you a high and low signal as you expect. Many data lines have a pull-up resister. In order to get a low signal, the ICs connected to the data line needed to drain current from the data line.

    This indicates that you can have 30 mA flowing constantly without overheating the IC. It does not mean that the current will be limited to 30 mA; it is still possible to have 60 mA flow through the IC (and probably smoke the chip). If you connect an LED, you will still need an current liniting resistor.
     
  3. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    I think you misunderstood both my questions and the datasheet.
    I drew a picture :

    LEDPIN.png


    Ok so we have current SOURCE and we have current SINK.
    The datasheet specifies about 10 times that it's a current SINK .
    So my question is that if I had it connected like I have it in the picture would it work?
    It would be sourcing current right? Because When it goes high , then the current flows from the pin which is at 5volts through the LED to ground .
    Or are they talking about electron current which flows opposite to conventional current flow ?

    Concerning the Constant current :

    These devices are supposed to output constant current adjusted by an external resistor. You confused it with a maximum constant output current.

    So my question is : If I connect and LED to a constant current source without a series resistor will that be okay?
    Assuming I adjust the current source to give say 10 ma using an external resistor to the appropriate pin on the driver.
     
    Last edited: Aug 25, 2013
  4. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    You're right, I was looking at the SOUT pin and not the LED output drivers (OUTn). Figure 3 on page 8 has the equivalent circuit for the output pin. Since it is an NPN transistor connected to ground (inside the IC), you need to use it as a low side driver.

    The circuit you drew will not work. Turn the LED around, add a current limiting resistor and connect it to Vcc.
     
    Shagas likes this.
  5. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    Yes, it is a constant current driver with a nominal sink current at each output determined by Riref thus you need to connect the LEDs with anode to the plus supply and cathode to the chip output, as shown in the datasheet.

    It won't work as you show in your picture.

    If you adjust the current to what you want by the value of Riref, then you don't need to add a resistor in series with the LEDs.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Your described is correct.

    It means that the Imax of current sink are 30mA, when you using a 20mA led then you still need to in series with a resistor to limited the current, otherwise the LED could be burned.

    About the input and output signal, Sin is the same with the output, you can see it on page 5, the internal structure of TLC5947, you can see the output waveform on page 16, they has labeled high=OFF, Low=ON, because the output need a Low level to drive the LED, so you have to input a Low level on the Sin pin.
     
    Last edited: Aug 25, 2013
  7. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74

    Yes , as I suspected ... Thanks

    Ok thanks I have all the info now

    I'm guessing that reverses my PWM values to make up for the inverted sink output.
     
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