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LM3429: Will it be damaged by shorting COMP pin to ground temporarily.?

treez
Intellectual 970 points
Other Parts Discussed in Thread: LM3429, UC2843, UCC28C43

Hello,

 

We are doing flashing LEDs, and every time we flash the leds back on, we want it to soft start back on, so with the LM3429, can we do this by simply shorting the COMP pin to ground  with a BJT?.....or would this damage the LM3429?

  • 0
    TI__Mastermind 25296 points

    why do not use nDIM pin, minded for this purpose?

    its duty cycle can span from 0 to 100% (so completely on / off)?

    Vincenzo

  • 0 in reply to Vincenzo Pizzolante
    Intellectual 970 points

    we  do not want to use the ndim pin, as when we flash back on the inductor current will overshoot and increase emc problems......we want to take the comp pin low, as if we do not , then it will be high as it can get and will command maximum duty cycle when we flash back on, and we dont want that...we need a soft flash back on.

     

    Is it ok to  ground the comp pin  by switching a BJT on with its collector connected to comp, and emitter to ground?

  • 0 in reply to treez
    TI__Mastermind 25296 points

    ok, if this is the case then short the comp won't help you (since it will act as you ground nDIM)....

    you can try, but I don't think you will achieve what you wish...

    if you wish a soft falsh on or simply reduce EMI then you have to speed down the FET transitions, just applying a gate resistor or a combination of resistor (one for  turn on and one for the turn off)...

    Vincenzo

  • 0 in reply to Vincenzo Pizzolante
    Intellectual 970 points

    As you know, there is more to EMC than the FET transition time......we need a soft flash back on....is there any way we can get this with LM3429?

  • 0 in reply to treez
    TI__Mastermind 25296 points

    what you are looking for is a Soft-Start feature in a device (LED driver) that normally hasn't...

    to achieve this feature you have to manipulare the signal coming from the current sense (refer to Analog Dimming Method section in datasheet): basically you have to force the device to regulate a non-constant current at the start-up, but a kind of ramp (when you enable it, for the time you decide it will last)...

    Vincenzo

  • 0 in reply to Vincenzo Pizzolante
    Intellectual 970 points

    surely shorting the comp pin to gnd does this?

     

    If comp pin voltage is low, then converter has low duty cycle, = soft start....this is like the ti.com  application  nores recomend for  soft starting ucc28c43/uc2843.......shorting the comp pin to gnd is an idea i got from ti.com (or rather unitrode) app notes...which are now pedalled by ti.com

  • 0 in reply to Vincenzo Pizzolante
    Prodigy 50 points

    HI，Vincenzo

    When my LM3429 plus external PWM signal, once the external voltage, LED brightness follows the input voltage changes, how to solve this problem?