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TLC59116 questions, 'ON' resistance, thermal performance, peak current.

Tom Smith
Expert 2930 points
Other Parts Discussed in Thread: TLC59116, TLC59116F

Questions;

1.  What is the 'on' resistance of the outputs in the TLC59116 (the current mode IC)?

2.  The TLC59116 (TSSOP package) gets hot and nearly goes into thermal shutdown when we run 16 strings of LEDs at 50 mA.  Is that to be expected?  We think we can get away with using the IC because we will be using the quad package which has higher dissipation.

3.  The problem described in 2. does not exist with the TLC59116F (the open drain IC) running the same 16 strings.  It runs cool.  Why is that?

4. What is the instantaneous peak current at the TLC59116 output when running 2 LEDs in series from 5V with 33 ohms in series, and when running 6 LEDs in series from 15V with 33 ohms in series?  Rext at the IC  is set to 360 ohms, which should be for an average current of about 50 mA.

-Tom-

  • 0
    TI__Guru 61695 points

    Hello Tom,

    1. Please check figure 9 in the datasheet. This figure gives the typical value for the voltage you need at least at the output to get the given current.

    2. This depends on the voltage you are using on the LEDs. If the LED anode voltage is 10V and the string consists of 2 LEDs with a forward voltage of 2V each, the IC has to dissipate 16* 6V * 50mA = 4.8W.
    The power dissipation capability of RHB package is better than PW package because RHB has a thermal pad that has to be soldered to the board. Please make sure that you have a good thermal connection to the GND plane on your board.

    3. TLC59116F does not regulate the output current, it just turns on the transistor fully therefore the power dissipation is minimized in this part, but be careful that your LED withstands the current. You need a series resistor at each output.

    4. Do you mean the peak current per channel or the peak current at the LED voltage? Normally the peak current per channel should be 50mA and for the IC it should be 16*50mA = 800mA.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Expert 2930 points

    1. Figure 9 in the data sheet I am looking at is titled TLC59116 40mA Per Channel Typical Application.  It's on page 13 of 35 in a document labelled SLDS157C – FEBRUARY 2008 – REVISED JUNE 2010. 
    Am I looking at the wrong data sheet?  What is the title of the figure 9 which you are referencing?

    2. This depends on the voltage you are using on the LEDs. If the LED anode voltage is 10V and the string consists of 2 LEDs with a forward voltage of 2V each, the IC has to dissipate 16* 6V * 50mA = 4.8W.

    That would be the calculation to use if the driver outputs were like adjustable resistors to ground, programmed to be current sinks.  But aren't they actually pulsed FETs to ground?  In that case I would be asking what is the 'on' resistance of these FETs when they are 'on'?  If the 'on' resistance was zero, for example, the dissipation in the TLC59116 outputs would be zero, even when the average current per channel was 50 mA for each of the 16 channels.  Of course a series resistor would be needed to limit the peak current in the LEDs, so I'm thinking that the drivers outputs have a non-zero 'on' resistance.  The outputs seem to be pulsed, as implied by the waveforms seen in Fig. 11 titled "Brightness and Group Dimming Signals".

    3.Yes, I understand that re the TLC59116F.  So, are you saying that the TLC59116 outputs are pulsed to adjust individual LED brightness, and that the current when the brightness is set to max is continuous, or nearly so, with the level being set by the programming resistor?
    If that is what you are saying, then I would ask what is the 'on' resistance of the FET outputs of the TLC59116 when the current is set to 50 mA and the brightness is set to max?

    4.I meant the peak current per channel, and you say that would be 50 mA when Rext is set to 360 ohms.  Does that mean that the outputs can be thought of as pulsed 50 mA current sinks when Rext is 360 ohms?

  • 0 in reply to Tom Smith
    Expert 2930 points

    I wanted to ping this again just as a reminder that I have additional questions.

    -Tom-

  • 0 in reply to Tom Smith
    TI__Guru 61695 points

    Hello Tom,

    1. The newest datasheet of TLC59116 is the following one: SLDS157D –FEBRUARY 2008–REVISED JULY 2011

    and this is the title of the figure I am refering to: Figure 9. Output Current vs Output Voltage

    2. For the current regulation, the current sinks are like regulated resistors, just for the PWM to regulate brightness or group dimming, the current sinks are pulsed.

    3. The on-resistance is dependent on the maximum set current and the voltage you have on VLED. As mentioned above for the maximum current, the current sinks act like regulated resistors.

    4. Yes, the resistor sets the maximum current and then the outputs are pulsed dependent on the individual brightness and the group dimming. Please check Figure 8 in the above datasheet for the right resistor setting.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Expert 2930 points

    Thank You.