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LM3553: PWM brightness control possibility?

Yoshwa
Expert 4050 points
Part Number: LM3553
Other Parts Discussed in Thread: LM3643, LM3644

Hi team,

Customer inquired PWM brightness control possibility by adding PWM signal to the FEN pin.

I is thought that the flash operation is repeated synchronizing to the high transition of FEP pin signal.

If so, are there any limitations for high and low period of PWM signal?

What kinds of issues should be considered?

  • 0
    TI__Mastermind 22745 points
    Hi Yoshwa,
    There are some detail parameters on the specs table, the minimum voltage for turen on FEN pin is 1V, and the maximum volatge for turn off FEN pin is 0.6V. And there is a detail description for FEN pin function on datasheet page 8:
    " The flash enable pin, FEN, provides an external method (non-I2C) for starting the flash pulse. When FEN is pulled
    high, logic '1', the flash current level defined through the I2C interface, will be delived to the Flash LED. If the FEN
    pin is driven low during the flash pulse, the flash event will stop. In the event that FEN is not pulled low during the
    flash pulse, the LM3553 will continue to deliver the flash current until the safety timer duration (set through the
    I2C interface) is reached.
    The LM3553 does not provide a fixed off-time after the flash pulse has ended. Most flash LED manufacturers
    require that the flash pulse duration be 10% of the total Flash cycle. Example: If the flash pulse duration is set to
    be 200 milliseconds (Flash Duration Code= 0011), the recommended off time for the LED would be 1.8 seconds.
    Please consult the LED manufacturers datasheet for exact timing requirements.
    If the LM3553 is placed in indicator mode or torch mode through the I2C interface and the FEN pin is pulled high
    and then low, at the end of the flash event, the LM3553 will return to the mode stored in the General Purpose
    Register.
    It is recommended that an external pull-down be placed between the FEN pin and GND to prevent unwanted LED
    flashing during system start-up due to unknown control logic states."

    Best Regards
    Sean
  • 0 in reply to Sean Zhou
    TI__Expert 4050 points

    Hi Sean,

    Many thanks for your comment.

     

    The following specification is customer’s practical application for LM3553.

    VIN = 3.3V

    LED : Vf=1.5V – 2.1V, 　If = about 400mA,　2-parallel×5-sleaze

    ON time = 0.5ms, period = 5.56ms, synchronizing 2-parallel,

     

    Is this possible for LM3553?

    If not, which product will be recommended?

  • 0 in reply to Yoshwa
    TI__Mastermind 22745 points
    Hi Yoshwa,
    I don't know whether LM3553 can support those short time, But I can recommed LM3643/LM3644 for your applicatiom. LM3643/Lm3644 can support IR mode, this mode can support very short flash time. Thanks.
    Best Regards
    Sean
  • 0 in reply to Sean Zhou
    TI__Expert 4050 points
    Hi Sean,

    Can LM3643/LM3644 drive VLED＝2.1V×5＝10.5V application?
  • 0 in reply to Yoshwa
    TI__Mastermind 22745 points
    Hi Yoshwa,
    No, LM3643/LM3644 can't support those high voltage application. You can apply an LM3553EVM to verify the short time pulse. I don't have the EVM on hand. Thanks,
    Best Regards
    Sean
  • 0 in reply to Sean Zhou
    TI__Mastermind 22745 points
    Hi
    I'll close this thread due to inactivity and if you have more questions please reply to it directly and it will open again. Please help to click "Resolve" if the problem is solved.
    Regards
    Sean