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tlc59116: max led power supply + power dissipation

Nicola Briox
Prodigy 100 points
Other Parts Discussed in Thread: TLC59116, TLC59116F

Hi i'm Nicola.

Maybe it's a stupid question, but can I use a voltage power supply (VLED) to feed my led array, of (let's say) 24V, or more, even though the max OUTx of the TLC is 17V (VLED>17V)?

In my setup, I have  an array of 7(or 8 also) led in series, the led Vf is 3.2v. Thus, the total voltage drop (VDROP) is 22.4V. 

As far as i can understand, since the OUTx of TLC59116 is in 0-17V with max 120mA, this means that the difference between the led power supply and the led voltage drop

has to be less than 17V, so VLED-VDROP <= 17V. It this true?

 

Another question related to this point is about the power dissipation calculation.

I cannot find any formula in the datasheet, I was thinking something like this

Ptot = (Vled-Vdrop)*Iout*N + Vcc*Icc

where N are the channel, Vcc and Icc are the maximum power supply and current for the TLC59116, respectively.

Can i assume a formula like this? Are there other factor i have to consider (i.e. i2c power, etc)?

My biggest concern is always about the relation between VLED, VDROP, and OUTx of the TLC.

 

Thank you,

nicola

 

 

  • 0
    TI__Guru 65470 points

    Hello Nicola,

    If you want to use a higher voltage than 17V for the LED voltage, I would recommend to use a transistor in series to the LEDs in front of the output. Please refer to application note SLVA280 for the circuit details. This circuit is not tested with TLC59116, but it should work the same way.

    When the LED is off, the voltage on the OUT pins will be higher than when the LEDs are on because the voltage drop is lower.

    Your power dissipation calculation is the one I would use.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 100 points

    Thank  you Brigitte for the answer,

    now i got the point of the n-channel mosfet.

    Just 2 more questions:

    1 - will PWM work with this configuration?if not, how can i do that with 'high' voltage VLED?

    1 - for the specific mosfet to use, are there specific guidelines to follow?

    Thank you,

    nicola

  • 0 in reply to Nicola Briox
    TI__Guru 65470 points

    Hello Nicola,

    1. The MOSFET does not change the general operation of the system, it just makes sure that the voltage on the OUTx pins rises above the maximum rating. PWM works with the MOSFET, but make sure that the series resistor does not limit the current too much because the MOSFET needs then some time to turn on. The right series resistor has to be tested in the specific system.

    2. The MOSFET has to withstand the LED voltage and current and has to have a lower threshold voltage than Vcc - R drop - Voutx to turn on when OUTx goes low.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 100 points

    Thank you for the support!

    But,

    1) - if my led series give me the assurance to drop enough volts to keep the OUTx of the TLC59116 under the 17V (also for very low forward voltage, thus when the leds are 'off'), can i avoid using the mosfet solution, right?

    I would like also to understand how the PWM work for this device.

    2) - How the device 'decides' the exact OUTx? The value of OUTx in the SLVA280, table 1, are automatically set by sensing the current flowing into the pin?

    3) - Can i assume the tlc59116 sink ALWAYS the current set with the Rext, also during the pmw (i.e., is the pwm working on the voltage)? If the current is always the same, can i put in series with the leds a resistor to drop the required volt (like I would do with the tlc59116f)? 

    thank you,

    nicola

  • 0 in reply to Nicola Briox
    TI__Guru 65470 points

    Hello Nicola,

    1. You have to make sure that in any case there will be never more than the absolute maximum rating on any of the IC pins. I leave it up to you, how you achieve this, but I cannot recommend to use a higher LED voltage because LED shorts could then damage the IC.

    2. The part permanently senses the current going into OUTx pins, each output pin is a current sink which regulates the current to meet the set value (as long as the voltage on the pin is high enough to meet this requirement).

    3. You can assume this, as long as the LEDs turn on and off fast enough.

    Best regards,
    Brigitte