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LM3555: how to configure I2C register to enable output?

Yuan Tan63
Expert 3280 points
Part Number: LM3555
Other Parts Discussed in Thread: LM3643, LM36010, LM36011, LM3644,

Hi,

customer found they cannot configure the OEN bit to 1 through I2C. they tried to change other bits in control register, it's OK.

can you help to give suggestion for that?

here is the sch:

they want to get the modes in red frame as below:

how should they do to make it? thanks!

Best regards,

Yuan

  • 0
    TI__Mastermind 22745 points

    Hi Yuan,

        Please check is there any fault when they try to write "1" to "OEN" bit. May I know what's the application, We recommend with new device for new project: LM3643/LM3644/LM36010/LM36011.

    BR

    Sean

  • 0 in reply to Sean Zhou
    Prodigy 30 points

    Hi,Sean

         We tried to set the OEN with 1, but no output voltage on VOUT and VLED.Also we read the OEN it still 0. But we set other register 0x03,0x01,0x02, we can write them successfully. So please look at the schematic, if our connnection is wrong or we need to send some command before set OEN? 

          And do you have the reference software setting the LED process, you can told us how to set the LED,let LED turn on. 

           By the way ,we connect 2 LEDs in series on VLED power, 

    Please advise.

  • 0 in reply to user5848156
    TI__Mastermind 22745 points

    Hi Yuan,

         As we have discussed on the phone, please follow two points to check the root cause:

    1. use the normal white LED as the load instead of the IR LED since you hvae found open or short fault;

    2. or serially connec a resistor with the IR led to test again.

    BR

    Sean

  • 0 in reply to Sean Zhou
    Prodigy 30 points

    Hi, Sean

           We have made more test with the LM3555 chip ,when we add a resistor between the ground and LEDs , The LM3555 can work, but we remove the resistor, it can't work again. So we think the issue is relate to the output of the chip need add some resistor, but it doesn't told any thing in the spec.Please help to confirm how to improve this.

  • 0 in reply to user5848156
    TI__Mastermind 22745 points

    Hi Sir,

        I believe that was caused by the IR LED since it has low forward voltage, I suggest you try with white LED test again without the resistor. That can help to identify whether the root cause is the IR LED or not.

    BR

    Sean

  • 0 in reply to Sean Zhou
    Prodigy 30 points

    Hi, Sean

           Do you konw what is the MInmum value of the forward voltage of teh LM3555? I will double check it again tomorrow.

    BR

    Changyu

  • 0 in reply to user5848156
    TI__Mastermind 22745 points

    Hi Changyu,

         The minimum threshold is about 0.8V. An SCP fault is triggered when the output voltage (VOUT) and/or the VLED pin does not reach 0.8 V in 0.5 ms.

    The short circuit condition is cleared when the output (VOUT) is allowed to reach its steady-state target and when the LED voltage rises above 0.8 V. A shorted output capacitor or a shorted LED could cause this fault to occur. This fault is reported to the SC fault bit in the Fault Register (bit6 in address 0x05).

    BR

    Sean