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LM3492(HC) overvoltage

Dick Veltman
Prodigy 75 points
Other Parts Discussed in Thread: LM3492HC, LM3492

It is not clear what happens if resistor Rfb1 is too high or even is an open circuit.

Q1 : What then becomes the value of Vout and can this (possibly too high) value damage the LM3492HC ?

Q2 : Is the overvoltage protection for protection of the LED string or for protection of the driver ? 

For a design with a LED string voltage of 50V the Rfb1 was choosen to be 390K. With this value and 16K2 for Rfb2,
the FB input voltage becomes 1.99V. This value is according to the recommended value of 1.05V en 2.0V
However with Rfb1 = 390K the Voutmax values becomes 2.8(1 + Rfb1/Rfb2) = 70.21V.

Q3 : Can this value damage the LM3492HC ?

If we choose the LM3492, then the formula for Voutmax is different and Voutmax becomes 2.5(1 + Rfb1/Rfb2) = 64.54V.
This value is lower than the absolute value of 67V according to the datasheet.


Q4 : Is it correct to state that the LM3492HC is not suited for this design and that using the LM3492 will be a better option ?


 

  • 0
    TI__Mastermind 25500 points
    The OVP is there to prevent damage to the device, not the LED string. If you exceed abs max you can cause damage so 70V would be too much. You would get a lower voltage using the LM3492, but that maximum of 2.5V was not characterized so I do not know what kind of tolerance it will have. The LM3492HC datasheet shows an alternative method of OVP where you connect a zener from VOUT to ILIM. Another method would be to use a zener across the upper feedback resistor so that as soon as it conducts it will pull FB up and force OVP.
  • 0 in reply to Clinton Jensen
    Prodigy 75 points

    Thanks for the advise.

    The alternative OVP method looks promising but I think it only works if there is a load, i.e. a LED string is connected to the driver.

    But what if there are  no LED strings connected to the driver ?

    Dou you agree that in case the upper resistor becomes "open circuit" the driver can also be damaged by a too high voltage ?

    Maybe a zener across the upper feedback resistor is a better option.

  • 0 in reply to Dick Veltman
    TI__Mastermind 25500 points

    Both methods should work, they just will work in a different way.

    A zener from VOUT to ILIM will just force the part to switch at minimum duty cycle keeping the output voltage lower. But I haven't tested it personally and I could see in a case of the input being close to the output so you are near minimum duty cycle anyway it perhaps may not work as intended.

    A zener across the upper resistor will put it in a burst mode type OVP where the part will switch until it hits OVP then stop until FB drops a bit then start switching and repeat. That should work regardless of input voltage and duty cycle.

  • 0 in reply to Clinton Jensen
    Prodigy 75 points

    A zener of 56V or 60V across the upper resistor seems to be a good solution.  However, how fast will the converter react on an overvoltage

    situation ? If this is too slow there will be a possibility that the FB input voltage becomes more than 5V for some time.

    Will it be necessary to protect the FB input for voltages > 5V 

  • 0 in reply to Dick Veltman
    TI__Mastermind 25500 points
    It will react very fast when FB hits 2.8V, just IC propagation delays which are very small. I do not think you need to worry about exceeding 5V on FB, but you could put a pad for a zener to ground just in case you feel you might have issues.