LT3942 - LED with Vf < 3V and PWM

Hello, 

I have designed a circuit using the LT3942 to drive several differents LEDs.
When I drive a Amber LED with Vf = 2.1V, I got a current peak of 570mA for 10µs then goes to 220mA.

Following curve shows signals when PWM signal rise:

Blue curve = V ISMON with Rshunt = 0.33o:
Red curve = PVOUT

Datasheet says that LT3942 is 0V to 36V Output Voltage Range.
My observation is that if the output voltage is less than 3V, the driver may not works properly with PWM command. In my case, the LED may be damaged because of over current (I max of the LED is 350mA).
What is the solution to avoid over current on a 2.1V LED ?

Thanks in advance for help.

Regards,

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  • 0
    •  Analog Employees 
    on Aug 5, 2020 5:34 PM 5 months ago

    Hi Dams,

    Are you using a PMOS in your design between the LED string and output of the LT3942? If so, can you specify which device you are using? A design like this will need a PMOS that is guaranteed to be fully enhanced by at least -2VGS. If the PMOS used isn't spec'd for low VGS turn on, it could be temporarily high impedance until it is turned on, resulting in the output overshooting. 

    If you are only driving single LEDs, I might suggest trying Si2329DS as your PMOS to see if this fixes your issue.

  • Hi, 

    Thanks for feedback.
    I'm using the PMOS SI2365EDS in my design to drive the LED.

    I'm assuming that the driver LT3942 is waiting for PVOUT>3V before driving the PWMTG output. Is a solution exist to drive a single LED with Vf<3V ?

    Regards,

  • +1
    •  Analog Employees 
    on Sep 3, 2020 2:26 AM 4 months ago in reply to Dams

    Hi Dams,

    LT3942 PWMTG requires a minimum VOUT of 3V to function. I believe that this is specified in the datasheet electrical characteristics table. If the LED used is lower than this voltage an external low-side NMOS must be used for dimming and will need to be driven directly rather than connecting to the PWMTG pin. If you can tolerate a little bit of lost power, can add a series resistor or silicon diode to add additional voltage drop to increase the effective forward voltage of the single LED.

Reply
  • +1
    •  Analog Employees 
    on Sep 3, 2020 2:26 AM 4 months ago in reply to Dams

    Hi Dams,

    LT3942 PWMTG requires a minimum VOUT of 3V to function. I believe that this is specified in the datasheet electrical characteristics table. If the LED used is lower than this voltage an external low-side NMOS must be used for dimming and will need to be driven directly rather than connecting to the PWMTG pin. If you can tolerate a little bit of lost power, can add a series resistor or silicon diode to add additional voltage drop to increase the effective forward voltage of the single LED.

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