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LM3409HV: LM3409HV efficiency issue with High Vin, drive one LED and ILED driving current set high to 3.5A ?

Sam Chan
Prodigy 120 points
Part Number: LM3409HV
Other Parts Discussed in Thread: LM3409

Hello, we have a question to ask,  here is, we are doing design high current LED drive PCB, customer assigned to use LM3409HV, the VIN = 48v , one LED only with PWM dimming control, VF LED = 3.5v , ILED=3A , for efficiency consideration, do we need to use a buck PWM controller to switch 48v VIN to 6v to provide to LM3409HV (LM3409HV Vin min. = 6v)  or we could input 48v to LM3409HV directly (due to 48v to drop to VF LED 3.5v, ILED 3.5A, is there efficiency and power dissipation issue of LM3409HV) ? 

Best regards,

Ekozy Sam

  • 0
    TI__Mastermind 28655 points

    Hello Sam,

    The LM3409HV is non-synchronous.  It will work with 48V input down to 3.5V output however the freewheel diode will be near 100% duty cycle so that will affect efficiency.  A synchronous LED current regulator can be more efficient.  There are a lot of designs out there that are similar to your application so it will work fine.  I would guess the efficiency to be in the 70's percentage range depending on the design, switching frequency and components used.

    Note that stepping down the voltage prior to using the LM3409 may not make a big difference in efficiency depending on the step-down buck converter used.

    One thing to watch in the design is to make sure the minimum on-time is long enough.  At this step-down ratio the duty cycle will be very low so this cannot operate at a high switching frequency.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 120 points

    Well noted and thanks Irwin for reply and suggestion :)

    Best Regards,