LT3743 Step-Down LED Driver OVP

I have received a couple of questions about OVP on the LT3743, so I am posting the latest question below to which I will post a reply:

Could you provide a detailed description of the feedback mechanism please? I am unsure of whether the voltage is regulated to a reference of 1V or 1V3. Is the Open-LED designed for in operation disconnects of the LED or during power up?



Fix typo in title.
[edited by: ARad at 6:28 PM (GMT 0) on 24 May 2019]
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  • +1
    •  Analog Employees 
    on May 24, 2019 6:26 PM

    Basically the LT3743 has two regulation loops:  a voltage regulation loop with FB as an input and a current regulation loop with SENSE+/SENSE- as the input.  When the LT3743 is driving an LED, the current regulation loop should be in control and the output voltage should be determined by the I-V characteristic of the LED load.  To ensure the current regulation loop is in control when driving an LED, the FB = 1V voltage regulation point in the case of the LT3743 should be programmed higher than the LED voltage.  For instance, LT3743 demonstration circuit DC1470A is designed to drive a diode with a forward voltage of 4.5V, but the FB regulation point is programmed to 6V. 

     

    To the best of my knowledge, the FB = 1.3V  OPEN LED threshold of the LT3743 will only be reached under unusual circumstances because in most cases either the LED voltage will set the output voltage so FB is less than 1V or the voltage regulation loop will try very hard to prevent FB from exceeding 1V and hence FB will not reach 1.3V where the protection occurs.

    I show the LT3743 block diagram in the image below for reference:

    LT3743 DS block_1.png

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  • +1
    •  Analog Employees 
    on May 24, 2019 6:26 PM

    Basically the LT3743 has two regulation loops:  a voltage regulation loop with FB as an input and a current regulation loop with SENSE+/SENSE- as the input.  When the LT3743 is driving an LED, the current regulation loop should be in control and the output voltage should be determined by the I-V characteristic of the LED load.  To ensure the current regulation loop is in control when driving an LED, the FB = 1V voltage regulation point in the case of the LT3743 should be programmed higher than the LED voltage.  For instance, LT3743 demonstration circuit DC1470A is designed to drive a diode with a forward voltage of 4.5V, but the FB regulation point is programmed to 6V. 

     

    To the best of my knowledge, the FB = 1.3V  OPEN LED threshold of the LT3743 will only be reached under unusual circumstances because in most cases either the LED voltage will set the output voltage so FB is less than 1V or the voltage regulation loop will try very hard to prevent FB from exceeding 1V and hence FB will not reach 1.3V where the protection occurs.

    I show the LT3743 block diagram in the image below for reference:

    LT3743 DS block_1.png

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