LT3476EUHF

Thank you for the post.  LED driver controllers similar to the LT3476 were not designed to PWM dim the LED brightness at repeat rates that are lower than around 100Hz.  If the repeat rate is lower than 100Hz, your eyes will see the blinking of the LEDs and that is not desirable in many cases.  Also for PWM dimming repeat rates that are slower than around 100Hz, the output capacitor will discharge too much when the LED is off and the over-discharged COUT will affect the accuracy of the current initially when the LED is turned back on.  The excerpt from the LT3476 datasheet section called Dimming Control shown in the image below discusses the minimum amount of time that the LT3476 needs when the PWM input is on or off, and those time minimums determine the maximum frequency of the PWM dimming repeat rate. 

Some recent LED drivers like the LT3922 perform better than the LT3476 when the LED is off a relatively long time because the LT3922 is designed for such "machine vision" applications. 

Hi

Thanks for the reply, we go through it and we have a doubt on negative transition of the PWM, means  toff of the duty cycle for PWM.

For 50 KHz PWM frequency what can be the minimum Toff time .Our switching cycle is 1us sec so for dimming minimum PWM on time should be 10us .there is a margin of 10us for the off time. We would like to know how low toff can be

Hi

Thanks for the reply, we go through it and we have a doubt on negative transition of the PWM, means  toff of the duty cycle for PWM.

For 50 KHz PWM frequency what can be the minimum Toff time .Our switching cycle is 1us sec so for dimming minimum PWM on time should be 10us .there is a margin of 10us for the off time. We would like to know how low toff can be

You can add an NMOS in series with the LED strings to turn the LEDs off very quickly.  Just drive the NMOS with your PWM signals.  I show the method in the image below and this method is referred to as the "disconnect" in the above text.  The text above also says that the LT3476 will start switching again after at least one microsecond when PWM goes high again.

I forgot to mention that the LT3909 is a good fit for your application.  The LT3909 is designed for two LED strings, so you will need two LT3909s if you have four LED strings.  You can order evaluation board DC2176A and verify the performance.