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LM3409: IADJ pin: hysteresis ?

Pascal PELLIZZONI
Prodigy 50 points
Part Number: LM3409

Hello everybody,

I'm evaluating the LM3409. I would want to control ouptput current by controling Iadj pin with an analog voltage. In real condition I remarked that this pin has a sort of hysteresis into it. I didn't find anything related to that behavior in the datasheet, so I simulated it in PSpice for TI.

The behavior is similar to what I've seen in reality. For that I just drawn this schematic with minimal configuration, with no oscillator:

We clearly see that for descending V(iadj), V(sw) turn off at V(iadj)=242mV and for ascending V(adj), V(sw) turn on at V(adj)=693mV, so an "hysteresis" of 450mV:

Can you confirm I'm seeing right ? Why such a high hysteresis ?

Thanks a lot in advance,

Pascal

  • 0
    TI__Mastermind 28655 points

    Hello Pascal,

    What you are seeing is not caused by Iadj, Iadj does not have hysteresis.  Do you have the rest of the schematic?  What is Coff connected to?  What is SW connected to?  Is the current sense resistor 47 ohms?  What is your load, is is resistive not LED?

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    TI__Mastermind 28655 points

    Hello Pascal,

    I'm not sure why your response doesn't show up here:

     

    Pascal PELLIZZONI replied to LM3409: IADJ pin: hysteresis ?.

    Hello,

    I simulated this partial schematic to take evidence of the hysteresis. The oscillator is not working, no inductor too. Just the PFET that turn on or off.

    47 Ohms resistor (R5) is the load. R4 is sense resistor.

    I did another simulation with the final schematic. Load is always a 47ohms resistor:

    I traced R5 current function of Viadj (so: I(R5) = f(V(IADJ) ):

    There is no oscillator in the LM3409.  You cannot simulate this the way you are.  The off-time is generated by the output voltage via a resistor and capacitor going to Coff.  If the voltage at Coff cannot rise to 1.243V it will go into maximum off-time mode.  The on-time is determine when the LM3409 hits peak current threshold, if it does not the Pfet gate drive will just stay on.  Please read the datasheet on how this part works.  It requires the external components and ability to have the output voltage rise above 1.243 volts.  This is an LED driver so driving a resistor load requires design work.

    I'm not sure how you replied to the previous post but it didn't show up here.

    Best  Regards,