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LM3405 at low Vin

uMin
Intellectual 860 points
Other Parts Discussed in Thread: LM3405XEVAL, LM3405

Hi,

 

In the user guide of LM3405XEVAL (snva222a), on first page, it states that this device can be configured to accept Vin from 3V to 5V.  But it also states that Vboost - Vsw has to be in the range 2.5V - 5.5V in order to ensure gate drive voltage.

Questions:

  1. Does this means at Vin=3V, the Vf of LED can only be 0.5V ?
  2. Can I drive a 3V /1A LED with LM3405 at Vin=3.5V or 4V ?
  3. For Vboost - Vsw, what is the definition of Vsw, is when the switch is turned on ?

 

Regards,

CM.

 

  • 0
    TI__Expert 5130 points

    Hello CM,

    Please see my answers below:

    1. Does this means at Vin=3V, the Vf of LED can only be 0.5V ?

    No. Actually there is a sentence after it states that Vboost - Vsw has to be in the range 2.5V - 5.5V in order to ensure gate drive voltage. In cases of low input voltages (3V to 5V) being used, the boost diode (D2) can be directly connected to VIN (R3 short, C4 and D3 not installed) to obtain sufficient gate drive voltage for best performance.

    1. Can I drive a 3V /1A LED with LM3405 at Vin=3.5V or 4V ?

    Yes, you definitely can do that. There are at least two method for your case. One is as I mentioned above. You can also refer to LM3405 data sheet pg.7&8 for the other method. I would recommend you to use 4V input as Vled may rise a little bit, using 3.5V input will make the duty cycle too large.

    1. For Vboost - Vsw, what is the definition of Vsw, is when the switch is turned on ?

    Vsw is the voltage of SW pin. It equals to Vin approximately during the switch on-time while equals to (–VD1) during the switch off-time.

    Best regards,

    Victor

  • 0 in reply to Victor Chen_HK
    Intellectual 860 points

    Hi VIctor,

     

    Thanks for the explanations.

    Yes, I think Vin=3.5 for 3V LED will probably hit the max duty cycle spec.

     

    Regards,

    CM.