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LM3413 / Operation with 1 string

S.Satoshi
Guru 21675 points
Other Parts Discussed in Thread: LM3431, LM3492HC

Hi,
I would like to ask about LM3431 to operate with 1 string.

1,
The LM3431EVB(snva309) recommend to use it with number of strings from 2 to 4.
Can we use LM3431 with 1 string with no problem?

2,
Our customer hope to use LM3431 with 2 strings mainly but also need to switch to 1 string.
They plan to disable 1 string by off the lower transistor forcibly by external circuit.
Please let us know the point need to case if they will use this configuration.
Or please let us know if you have other idea to disable string individually.

Best Regards,

  • 0
    TI__Intellectual 2540 points

    Satoshi-san,

    First, I would like to suggest looking at LM3492HC.  It is a 2-string driver with boost (a bit similar to LM3431).  Based on the information you have provided, I think the LM3492HC will be easier for your customer to achieve their goal.  Please have a look at it.

    Answering your questions on LM3431:

    Yes, one string operation is possible.  When a LED string is disconnected, the emitter voltage of the external BJT drops and the LM3431 pushes the base current to the maximum in order to maintain current regulation.  The external BJT must be switched off to avoid pulling the CFB pin voltage low when a output channel  is not used.  This can be done by pulling the SNSx pin voltage up to exceed its internal reference voltage, but you need an extra resistor between the SNSx pin and the current sensing resistor.

    Regards,

    John

  • 0 in reply to John Perry
    Guru 21675 points

    Hi John-san,

    Thank you for your response.

    Could you please see attached file and check my understanding is correct?

    Regarding LM3492HC, I also recommend this device to customer. Thank you!

    Best Regards,

    Sonoki

    LM3414 LED string disconnect.pdf
  • 0 in reply to S.Satoshi
    TI__Intellectual 2540 points

    Satoshi-san,

    The suggestion from our applications engineer is like this:

    "Pulling up the SNSx pin voltage will shut down the current regulator of an unused channel without hitting fault protection."

     

    Regards,

    John

  • 0 in reply to John Perry
    Guru 21675 points

    Hi John-san,

    I'm sorry but I will start new post for this request because I used incorrect part number in title.