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1W LED driver using TL4242

prashant rane
Prodigy 10 points
Other Parts Discussed in Thread: TL4242

Hi,

I wanted to know, whether a circuit designed with TL4242, in Enable (non PWM) mode, can work on a varying voltage inputs (6V, 9V, 12V, 18V).

I want to keep the same circuit and components and operate on  4 diff voltages. Will the single 1W LEd survive?

Shall I connect 'ST' and 'PWM' to Vcc through 10k resistor, if I wanted to bypass 1block and microcontroller as shown in the "Figure 5 Application circuit" in the datasheet?

regrds,

Prashant

  • 0
    TI__Genius 14890 points

    Prashant,

    The TL4242 is a constant current LED Driver.  As a result, the LED will receive the same current no matter what the input voltage is set to.  However, the extra voltage (and power) will be across the TL4242.  As a result, you will need to do some power management (Thermal).  For example, if your current is 250mA and your LED forward voltage is 4V, then the LED power is 1W.  With an input to the TL4242 of 6V, the TL4242 power is 6V-4V-170mV (R-REF) = 1.83V.  The resulting power is 1.83V * 250mA = 0.4575W.  If the input voltage is increased to 9V, the LED forward voltage, and R-Ref voltage remain constant so the extra voltage (and power) is across the TL4242.  The power dissipated in the TL4242 becomes (9V-4V-0.17V) * 250mA = 1.2W.  Care must be taken to avoid too much power dissipation on the TL4242.  Using series resistors would be one way to reduce the power dissipated on the TL4242.

    Connecting ST and PWM to VIN through a 10K resistor would work great.

    Regards,

    Stacey

  • 0 in reply to Dick Stacey
    Prodigy 10 points

    Hi!

    Two questions to your answer...

    what for do you recomend to connect the ST and PWM with an 10K to Vin?

    In My circuit the PWM is conected throug 180k to Vin.

    Second question:

    I measure even with 0R to Vin only 33µA to PWM-Pin... Why? In your datasheet you mention nominal 200µA up to 500µA @5V.

    I have already conected the PWM to 5V but the Ipwm just shrinks to 4µA...

    Thank you so much for answering this questions!

    Regards

    Simon

  • 0 in reply to Simon Brylka
    TI__Genius 14890 points

    Simon,

    The PWM pin has a pull-down to assure that the output (Q) is disabled unless intentionally pulled high.  If the pin is left floating, it will pull low and no current will go through the device.  There are several things that impact the current into the PWM pin.  Among the influencing factors are:  PWM voltage, VIN voltage, LED current, ambient temperature, die temperature.  Additionally, the manufacturing process will have an impact on the pull-down current.  When we create a spec for the datasheet, we take all of these factors into account to attempt to let the user know how their system may interact under worst case situations (and typical situations).

    I have tested a device in the lab at room temperature and found at PWM = 5V, the current is below 3uA.  When the voltage on the "I" pin increases, the current decreases.  When the  voltage on the PWM pin increase the current increases.

    The choice of a pull-up resistor is suggested to reduce the current into the PWM pin and thereby decreasing the quiescent current of the system.  Larger resistors can be used.

    Regards,

    Dick