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LM3410: Sizing OVP Protection Zener + Resistor Path

James Morrison30
Intellectual 560 points
Part Number: LM3410

Hello,

Could someone provide a bit more guidance on the protection circuit mentioned in section 8.1.1.7 of the LM3410 datasheet?

My LED string has Vf=8-9V and we'll be driving it with 500mA normally.  I calculate R(fb) = 390mOhm for the normal case.

If I use a 10V zener for D2 in Figure 14, it will never fire under normal circumstances.  Then R3+R1 will drop the 10V (assuming high-impedance into VFB pin of LM3410).  It we cap at 500mA then R3 must be ~19.6ohms.  That is OK, but that resistor will be dissipating 4.9W!  I don't see any way around that.  Any ideas without adding some significant circuitry (opamps, etc)?

James.

  • 0
    TI__Mastermind 28655 points

    Hello,

    See 8.2.6 as an example.  The current sense is just 390 mohm.  The added resistor, R3, and zener, D2, are out of the circuit unless the output reaches point where the zener starts conducting.  The added R3 is to prevent excess current in the current sense resistor. It allows fb to be pulled up basically shutting down the converter until fb drops back to regulation.  I would use a higher voltage zener in your case because zener diodes start conducting at a lower voltage than their rating.  You have to look at the datasheet of the zener for this.  If it starts supplying current to fb the current regulation set-point will drop.

    R3 + R1 does not drop 10V, the zener just pulls fb up above 190 mV which causes the LM3410 to adjust its internal error amplifier which will cause the LM3410 to stop switching until the fb node drops.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Intellectual 560 points

    Thanks Irwin. Right, zener drops the voltage, of course.  So how do we size R3?

  • 0 in reply to James Morrison30
    TI__Mastermind 28655 points

    Hello,

    Low enough the zener doesn't pull it up the voltage under all operating conditions, high enough that the peak power isn't too high.  The example in the datasheet has a value, I'd start with that.  R3 = 100 ohms.

    Best Regards,