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TL4242 with very low current

Frederic Ponceau
Prodigy 20 points
Other Parts Discussed in Thread: TL4242

Hello,

How to use the TL4242 for low LED current (<10 ma)?

The equation given in the datasheet (I=0.177V/Rext) is good only for Rext up to 10 ohms?

What is the equation for higher value of Rext?

What is the relationship between the PWM duty cycle and the current constant?

Can we control the Ref pin through a DAC to select the current value?

About my poject:

I want to use the TL4242 to control one backlight LED. The PWM input is use to turn on and off the LED (signal coming from the display driver). So the plan is to control the current through the LED using the Ref pin input. I need to adjust the current in the single digit range (eg nominal current is 2ma with possible 1ma increments up to 10/15 ma).

Am I using the right IC?

Thank You

Frederic Ponceau

  • 0
    TI__Expert 5275 points

    Hi Frederic,

     

    In regards to the I=0.177V/Rref equation, the device regulates the output current by sensing the voltage across the Rref resistor. In theory, this linear relationship would apply for any value of Rref.

    I believe the reason for 10ohms max recommendation is for accuracy.  +/-1% change in 100ohms will result in much more current variation than +/-1% of 1ohm.  This 10ohms max recommendation translates to a minimum current recommendation of 17.7mA.  While I don't think there will be catastrophic results of a higher Rref value, the accuracy will almost certainly degrade. 

     

    The PWM duty cycle is directly proportional to the output current.  So you might be able set the default current to 18mA and then adjust the duty cycle to select the current (~10% = ~1.8mA)  (~15% = ~2.7mA).

    As for driving Ref pin with a DAC.  I don't think this will work too well.  Since the device tries to regulate this pin to 177mV, forcing the voltage higher or lower will cause the driver to turn completely on or completely off.

     

    Regards,

    Adam Hoover

    DLS Applications

  • 0 in reply to Adam Hoover
    Prodigy 20 points

    Hi Adam,

    Thank you.

    I am not sure I follow you when saying increasing the resistor value would increase the power dissipation.

    I would think the power dissipation would decrease (P=U^2/R).

    My plan was to double the resistor value (so 20 ohm), so would give me a larger PWM range for fine tuning of the current.my current would be 9mA with 100% PWM and 2mA with a 25% PWM.

    This would give my end user a little more flexibility as far as adjusting the brightness.

    Regards,

    Frederic Ponceau