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LM3445: Application note for PWM Dimming

tateo yamashiro
Guru 10890 points
Part Number: LM3445
Other Parts Discussed in Thread: TPS92075,

Hi,

We got a question from the customer about LM3445.
Could you help us?

[Question]
Is there any application notes for PWM Dimming? They would like to make sure how to design the PWM dimming circuit by LM3445.

Best Regards,
tateo

  • 0
    TI__Mastermind 28655 points
    Hello Tateo,

    How do they plan on using the LM3445, as a valley fill buck convertor, non-isolated, flyback? What topology do they plan on using? What output power? What input voltage range? What is the output voltage/current? Is there Power factor requirements? EMI?

    Regards,
  • 0 in reply to Irwin Nederbragt
    Guru 10890 points

    Sorry for the delayed reply. Their condition is below.

    Vin=AC100V
    VLED=37V
    ILED=150mA
    Topology=non-Isolated Buck
    Dimming=Triac Dimmar and PWM Dimmer

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Mastermind 28655 points

    Hello Tateo,

    There are two designs available, the LM3445EVM-695 and AN-2061.  The EVM is the valley fill.  Do you know which one they would be interested in for their design?  Note that 5.55 watts is low power and will make compatibility with triac dimmers difficult if it doesn't maintain the hold current of the triac.  There is another part, the TPS92075 that is a buck converter intended for triac dimming with power factor correction.

    The valley fill board has circuits added to keep hold current in place when the valley fill capacitors are providing the current to the LEDs.  The datasheet does not have the calculations for this since it was done before the board design.

    BR,

  • 0 in reply to Irwin Nederbragt
    Guru 10890 points

    Hi, Thank you for your support. Let me confirm one thing. When they input the PWM pulse to DIM pin, can they input the pulse directly? If external circuit is required, could you provide the solution?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Mastermind 28655 points

    Hello Tateo,

    To drive the DIM pin FLTR1 needs to be pulled above 4.9V (typ) which causes the ramp generator to be tri-stated disabling the DIM decoder.  This allows the DIM pin to accept a PWM signal.

    The datasheet shows how this operates in the master/slave section, starting at 7.3.14 on page 17, and more description in section 7.3.9 on page 13.

    The PWM will need to be inverted, 100% high is low output, 0% duty cycle PWM is full output current.  Thresholds are 0.9V and 3.15V for worst case tolerance.

    BR,