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LT3092 problem...

Discussion in 'General Electronics Discussion' started by djape, Jul 3, 2013.

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  1. djape

    djape

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    Jul 3, 2013
    Hi everyone,
    I am trying to get stabille em field in inductor so what i need is inductor and some current source to drive that inductor.The inductor is wounded and have cca 2000 turns, which in this case isn`t so important.
    Problem i have in this case is with current source.I need stabile and constant current source, so i used lt3092-programmabile cs which is capable to produce up to 200mA. Several of them connected parallel will produce, or should produce larger current equal to summary of current each of them produce.And that is what is this "thing" doing. But during in time, after several moments current which LT3092 is producing suddenly starts to drop.It is not constant any more and also magnetic field in inductor also drops.I have tried to substitute inductive resistance with equivalent resistor and the same thing is happening.
    I have several of lt3092 and tried them on external board.On the start current they produce is stabile and gets rapidly low after several moments.I used two resistor configuration to set desirable current, which is available in datasheet.Current each of them produce is 180mA.
    Does anyone have some idea about this or from yours experience with this ic`s?
    Thanks
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Are the LT3092's getting hot? (As the datasheet notes, they have overtemp shutdown)

    What package are they in?

    What is the voltage dropped across them when in operation? (Alternatively: What is the supply voltage? And, what is the resistance of the inductor?)

    What sort of heatsink (if any) are you using?
     
  3. djape

    djape

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    Jul 3, 2013
    They are sot223 package. I supply them with external adjustable supply.Externall voltage which supply them is between 12-36V. When 12v is input output is something about 11,5V and when 36V is applied to them output is about 35,something V. I should drive them with 36 V.
    Yes, they get hot and there is no heat sink or some copper are on pcb on which they are soldered direct. Resistance of inductor is about 40 ohms and the same resistance of resistor..
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The voltage measurements -- are they with the load attached or not? What is the voltage across the load?

    If they are SOT-23 devices and they're getting hot, that will be the reason for them going into shutdown.

    SOT-23 devices often have a thermal resistance junction to air of around 450 degC/W. Even if they have only 1V across them, at 200mA they will dissipate 200mW and their junction will rise by 90C. And that may be sufficient to trigger their thermal shutdown.

    If the inductor has a known resistance and you have an adjustable voltage supply, the easiest method to set the current is to use Ohm's law to calculate the required voltage.

    If you must use a current regulator then get a larger device and use an appropriate heatsink.
     
  5. djape

    djape

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    Jul 3, 2013
    Voltage across the load is 6.8V on out current of 185mA.Measured resitor is 37.4ohms. Voltage between out and gnd when the cs is driving is about 6,88V at any voltage suplly
    So, voltage across the load is the same voltage between out and gnd.(path is cs-load-gnd)
    I must say that i noticed that there is no current drop when it is driven by 20V and in the same time cooled by fan.
    I suppose it would be much better if there is copper plane placed or any kind of heat sink.
    Still, there are a few watts power dissipation on ic`s, and in datasheet stay that sot223 package have 20degC/W thermal coefficient; its temperature will be:

    (Vsupp-Vout)*I=(Vsup-6.8V)*0.18=0.936 W Temp=Tambient+20*p=25+20*0.936=40degC
    5.25 W Temp=Tamb+20*p=25+20*5.25=125degC

    This is rough calculation but the most possible problem lies on its temperature dissipation...
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    SOT223 have 20 degC/W or whatever when attached to a large area of copper and with heaps of vias to the other side of the board. Recall also that you said SOT-23.

    Again, this is a fixed resistance. Don't bother with a current regulator, just get an adjustable voltage regulator. They're far more common, cheap, and easily available in packages that will dissipate more power.

    I would possibly go as far as to use a switchmode power supply since that is going to have far fewer losses.

    Also note that if you're going to use linear regulators then you'll benefit greatly from reducing the input voltage so that the regulators have the least amount of power to dissipate. In your case I'd recommend about 2V across the device plus the drop across the sense resistor, plus the voltage across the load. (That would mean about 9V).

    Is there a reason you chose these devices? You seem to be operating them near their current limit and intend to also operate them close to their voltage limit. This is almost guaranteed to be way beyond their ability to operate in still air.
     
  7. djape

    djape

    4
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    Jul 3, 2013
    I have had already ic`s.
    As i sad before i need to generate constant magnetic field in inductor. To do that i need constant current source...
    I have decreased supply to 15V and when supply them from external power supply they dont stop working but when i connect them on transformer via voltage regulator on the same voltage (15.5V) current is decreasing, as it was before with greater voltage.
    Circuit is behaving different on 15V when are connected on power supply and on transformer.
    Power of transf. is 50 VA, while total consumption of power is about 10W of LT3092, plus the power disipation on voltage regulator.
    Transformator is 230V/30V, and after capacitor there is 42V; so the disipation on voltage regulator is (42-15)*I=25*(~1A)=20+W
    Power of transformer is sufficient for all power losses in circuit..
    Now i am confused why is there difference between supplying from external power supply and from transformer?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Is your 15V regulator going into thermal shutdown now?

    See if the 15V from that regulator falls or stays constant. Is that regulator getting hot?
     
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