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TL4242 / switching the shunt resistor

Ryuji
Guru 20090 points
Hello,
 
Please see the attached file.
 
 
My customer change the LED current by switching the shunt resistor of REFpin by FET.
In this case are there any problem ?
 
I believe that , there is no problem about LED constant current.
But, output voltage has droop or overshoot due to dynamic load change by FET load switchnig.
If Vin voltage is closest to LED Vf, LED may not light by transient response.
 
Best Regards,
Ryuji Asaka
  • 0
    TI__Intellectual 1990 points

    Hi Ryuji,

    I am curious as to why they are choosing to adjust the LED current like this? A similar function is achieved via the PWM input pin already on this device. I would recommend that they adjust their current through the duty cycle of the PWM rather than using a FET like they are doing now.

     

    Thanks,

    Nate

  • 0 in reply to Nathaniel Wilson
    TI__Expert 5275 points

    Hi Ryuji,

     

    I think what would happen is the opposite of what you have suggested.   There will probably be minimal voltage droop or overshoot due to the load change given it is in the ~1ohm range.    Because this resistor is used for current sense instead of current limiting, the output current will start the change to match the new reference value.  Since there is a finite delay from sensing the current to adjusting it, there could be some current droops or overshoot when changing Rref.

    If overshoot/droop become a problem, a resistor can be added to the gate of the FET (shown below) to slow the transition time and better control the slew rate.

    Let me know if you have any more questions.

     

    Best Regards,

    Adam Hoover

    Drivers and Load Switch Applications Team

  • 0 in reply to Adam Hoover
    Guru 20090 points
    Hello Both,
     
    Thank you for the reply.
    I understood.
     
    Since our customer can't use PWM dimming function for avoiding pwm switching noise, they considered changing the current by FET.
     
    Best Regards,
    Ryuji Asaka