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LM3435: i2c reg, false fault

Tal Berger
Prodigy 40 points
Part Number: LM3435

Hi,

On LM3435 when setting low values on I2c register for setting the RGB LEDs (Reg 00H~03H, less then 14 on red, and less then 2 on Green or Blue) , we got false fault on reg 07H.

Do you familiar with that?

How i can avoid this?

Thanks

Tal

  • 0
    TI__Mastermind 28655 points
    Hello Tal,

    What are the faults being reported on reg 07H?

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Hi

    I saw that the bits of short are '1'.

    Tal

  • 0 in reply to Tal Berger
    TI__Mastermind 28655 points

    Hello Tal,

    Can you measure the voltage on the drive pins of the LEDs when this is happening? Also, is there any capacitance or resistance across the LEDs in your design? You may have to just disable the Fault when running low current.

    The Vf of a red led is lower than the other two colors, hence the higher number when it first faults.  I'm thinking the voltage is getting low enough to trip the fault.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Hi 

    Thanks for the info.

    on LM3435 datasheet , on page 9 at the block diagram, I saw VFB. but I can not find its value.

    Do you know the VFB value?

    ---

    I don't have cap or res across the LEDs. and I disabled the fault alarm when I set low values.

    Thanks

    Tal

  • 0 in reply to Tal Berger
    TI__Mastermind 28655 points
    Hello,

    VFB is for keeping Vout from going too high. Actual regulation is current regulation and three capacitors store the channel voltages when switched between channels: "In addition, POT control requires no external compensation and achieves fast transient response of the output
    voltage changes that perfectly matches the requirements of a sequential RGB LED driver. The POT flyback
    converter only operates at Continuous Conduction Mode. Dead-time between main MOSFET and synchronous
    MOSFET switching is adaptively controlled by a minimum non-overlap timer to prevent current shoot through.
    Initial VOUT will be regulated at around 3.2V to 3.5V above VIN before any control signals being turned on.
    Three small capacitors connected to CR, CG and CB pins are charged by an internal current source and act as
    soft-start capacitors of the flyback converter during start-up. Once initial voltage of VOUT is settled, the
    capacitors will be used as a memory element to store the VOUT information for each channel respectively. This
    information will be used for VOUT regulation of respective LED channel during channel switching. In between the
    channel switching, a small I2C programmable blank out time of 5 μs to 35 μs is inserted so that the LED current
    is available after the correct VOUT for the color is stabilized. This control scheme ensures the minimal voltage
    headroom for different color LED and hence best conversion efficiency can be achieved."

    If you have a chance to look at some of the waveforms perhaps you will find what is causing the faulting. I would start with VOL and others to see what could be causing the faults. If it works with the faults disabled and you are okay with it that can be one way around it. Another may be to leave the current level higher but reduce the PWM pulse widith.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Hi

    Thanks for the info.

    I will work with fault disable, when I will set low current.

    I can't change the PWM (it set from outside)

    Tal