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LM3429 LED driver.....Can dimming FET be a BJT instead?

treez
Intellectual 970 points
Other Parts Discussed in Thread: LM3429

Hello, We  are doing a buckboost led driver using LM3429. (15W) We want to flash the leds on and off at ~2Hz. We intend to use the nDIM pin to do this. ... though can we use a BJT instead of a FET  to take nDIM low and turn the leds off? (example shown on page 14, figure 13 of datasheet) http://www.ti.com/lit/ds/symlink/lm3429.pdf

  • 0
    TI__Mastermind 25546 points

    actually the pin is logic level, so you can drive it direcly from a logic gate / GPIO but, in order to reply your question,

    yes, as long as nDIM pin voltage is withing the reccomanded range of value, you can use either a BJT or a FET...

    Vincenzo

  • 0 in reply to Vincenzo Pizzolante
    Intellectual 970 points

    in the eval document , the dimming fet is the divider side of the series resistor,  -wheras in the datasheet its the other side of this resistor,  why is this?

  • 0 in reply to treez
    TI__Mastermind 25546 points

    actually there is no big difference.

    In datasheet version it is guatantee that nDIM voltage is below the turn off threshold, since the pin is directly shorted to GND.

    In the EVM version the pin is not directly connected to GND, but is grounded via R4: since the pin can source up to 30uA it must be

    ( R4 + Q3_RdsON ) x 30uA < 1.1V      [very worst case]

    that limits the range of possible values for R4. On EVM R4 is about 17K that means the voltage on nDIM is less than 510mV (the fet is ignored) whan Q3 is closed, so no problem at all.

    Vincenzo