LT3942 - LED with Vf < 3V and PWM

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,

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.

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.