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LM3447: Starting LM3447 with fault

Philippe Hebert
Prodigy 45 points
Part Number: LM3447

Hi, we purchase the LM3447-A19-120VEVM and we modify to our need (new transformer and related part). We try to make it start but the IC start in 812ms fault at Vin 36Vac rms.

VCC is stable at 11.5V.

Vbias is around 14V.

COMP have triangle ramp upto 336mV then off

Vaux have a pulse of 672mV peak

Gate pulse then off for 812ms

FLT1 have square pulse of 1.7V when around 36Vac rms in, then continously 1.7V output (56Vac in rms)

I can provided you my schematic and my transformer information by email if you need it.

Please let me know what I can do to resolve that starting issue.

thanks

  • 0
    TI__Mastermind 28655 points
    Hello,

    812 mS is the fault timer in the LM3447. There are two OVP functions, one is on VCC at 18.9 volts, the other on the AUX pin when it hits 1.75 volts during the MOSFET off-time. There is also over current protection if it exceeds the 275 mV threshold. I would check to see if any of these are causing the IC to go into fault mode.

    Thanks,
  • 0 in reply to Irwin Nederbragt
    Prodigy 45 points
    We try to adjust the R9, R10 but is seem impossible to get the chip to start correctly. The Aux or the VCC is tripping.
    Is it possible to send you our design by email and tell us if something is wrong?

    best regards,
  • 0 in reply to Philippe Hebert
    Prodigy 45 points
    Hi, we got it started by removing the D6 that remove the auxilary winding of the vcc power. We plan to add zener on the VCC to protect it later. We got the led light but we are not able to control the current near our setting. We adjust the Rsn but still have a big variation of 25mA from 90Vac in to 120Vac in. What do you recommand to adjust it. How the IC control the current. I know the OCP is 275mV on the Isns pin but what is the setpoint he try to control?
  • 0 in reply to Philippe Hebert
    TI__Mastermind 28655 points

    Hello,

    The LM3447 regulates input power.  If the LED string remains a constant voltage the LED string current should remain constant.  The current sense resistor is for safety (excess current in the MOSFET), it is not used for controlling LED current.  Did you go through the datasheet and calculate your new design?  Have you looked at waveforms to make sure the LM3447 is operating correct.  You shouldn't need a zener on VCC if the transformer is designed right for the application.

    What did you change from the original design to this design?  Power level?  Output voltage/current?  Transformer changes?  Where did the transformer come from?

    Thanks,

  • 0 in reply to Irwin Nederbragt
    Prodigy 45 points
    We provide to Irwin our current output that was not triangle. He found that the secondary output on the Ti evaluation board schematic was wrong. We reverse the output, as he suggest, and everything is fine. Great catch. Thanks for your support.