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Original question:

LM3409

LM3409HV: LM3409

Ambar Pakhira
Prodigy 80 points
Part Number: LM3409HV
Other Parts Discussed in Thread: LM3409

Hi,

We are planning to use LM3409HV PFET LED driver in one of our application. We have an abnormal transient survival requirement of 80V for 100msec. However, we noticed that Vin pin of controller max. rating is 76V.

Will you suggest any simple passive solution like zener clamp with current limiting resistor feeding Vin pin of controller to protect it during this transient? We are not tying UVLO & EN pin directly to raw power, so we have to protect only Vin pin.

Please advise on simple passive solution to ride through 80V transient for 100msec.

Thanks,

Ambar

  • 0
    TI__Mastermind 28655 points
    Hello Ambar,

    I need more information than this. What is allowable for resistance in the power line? Zener diodes or TVS diodes are not very accurate and have a fairly soft knee. As an example, if you clamp at 65V with a 1 ohm series resistor and 80V for 100 mS you will have 15A of current and 225W of power. The energy of this pulse for the resistor is 225W * 0.1 second or 22.5J. The TVS would see 65V * 15A = 975W for 0.1 second or 97.5J. This will also have 1 ohm of impedance in series with the LM3409 which will affect efficiency and low voltage operation depending on your power level and lowest operating input voltage. I checked and there are through hole resistors that can handle this if that is the route you want to go.

    What is your nominal input voltage range?
    Lowest input voltage? Highest operating input voltage without the transient? Output voltage? Output current? How often does this transient occur? Can it shut off during the transient?

    You can also use an active circuit that limits the input voltage to the LM3409 a power transistor or MOSFET and several components. There are also IC that can protect for this transient.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 80 points
    Hi Irwin,

    It is permissible to turn off the circuitry during this transient but it should get enabled after the transient passes and input voltage returns to normal level. We prefer to stick to passive circuit solution but if you have suggestion of active transient protection circuit we can consider it if you provide the details.

    If we add a current limiting resistor and 70V zener feeding only the Vin pin of the controller and rest of the power circuitry can handle 80V transient voltage, will this solution work?

    Regards,
    Ambar
  • 0 in reply to Ambar Pakhira
    TI__Mastermind 28655 points
    Hello Ambar,

    If you can design the circuit to handle the transient and the voltage is limited to below abs max of the LM3409 it should work. I would try and find out what the specifications of the transient are. The other issue is if you clamp in this method the current from the power source will be very high for the transient duration possibly causing a fault in a fuse or other protection device. They is a system level issue, you need to know the whole system to know how to tackle the problem.

    BR,