About LT3475 Parallel Connected 3A Output Circuit

Hi,

I would like to use LT3475.

So, I have a question. Please help me.

Question:

I want to use the LT3475 with 3A output.

However, there is no description other than the circuit example, and the design has stopped.

I want to light the LED at intervals of a few microseconds,

but I think that it is necessary to build a circuit like D/S p.15 to do that.

https://www.analog.com/media/en/technical-documentation/data-sheets/3475fb.pdf#page=15

Even if the same PWM signal is input to PWM1 and PWM2,
"Jitter occurs in the pulse output of the LED current, making it difficult to pulsate the LED in a few microseconds."

Is such a phenomenon likely to occur?

It seems that LTspice is working properly.

Best Regards,

Naohisa.

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  • Hi Naohisa,

    Dimming at microsecond intervals is extremely fast for PWM dimming. If you consider a pulse with a 2 microsecond period, that's 500kHz dimming. Even operating the LT3475 at 2MHz switching frequency there wont be many switching cycles to regulate the output during PWM on time.

    I'm also uncertain that this part works well for paralleled outputs due to its antiphase switching. To properly parallel the outputs of two LED driver channels, you would need to synchronize the switching with 0° phase between the two outputs. The "jitter" that you are seeing is most likely due to the antiphase switching with very short on-time dimming. This is caused by the inductor current not starting at the same point for both channels when the PWM pin is pulled high.

    I would recommend taking a look at LT3932-1. I have used two of these devices in parallel for 3A output in the past, and they are capable of operating at 2MHz switching frequency to help with your dimming requirement. 

Reply
  • Hi Naohisa,

    Dimming at microsecond intervals is extremely fast for PWM dimming. If you consider a pulse with a 2 microsecond period, that's 500kHz dimming. Even operating the LT3475 at 2MHz switching frequency there wont be many switching cycles to regulate the output during PWM on time.

    I'm also uncertain that this part works well for paralleled outputs due to its antiphase switching. To properly parallel the outputs of two LED driver channels, you would need to synchronize the switching with 0° phase between the two outputs. The "jitter" that you are seeing is most likely due to the antiphase switching with very short on-time dimming. This is caused by the inductor current not starting at the same point for both channels when the PWM pin is pulled high.

    I would recommend taking a look at LT3932-1. I have used two of these devices in parallel for 3A output in the past, and they are capable of operating at 2MHz switching frequency to help with your dimming requirement. 

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