LT1009 open loop simulation

Good Morning!!

I am kindly asking the community to understand what can happen when LT1009 precision reference is simulated in open loop. 

You can see the schematic simulated below:

And the result I get is:

I'm expecting the behaviour showed in the image below. Whenever the input signal (blue) overpasses reference voltage (2.5V) the output (red) goes from 5V to 2.5V (Vref)

It's quite strange since it is correctly working in closed loop in other simulations I have but I can't figure out what's happening. The current usage it has is as a comparator and in fact, in the real PCB the circuit is properly working.

Thanks a lot in advance for your kind support.

Best Regards,

Jose

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  • Hi Jose,

    I verified the LTSpice schematic you shared on bench with Rbias and Radj both set to 1k. As V4/ADJ pin supply voltage increase the VOUT also increase. This observation does correlate with LTSpice LT1009 model and design schematic shown in data-sheet. 

    looking at LT1009 design schematic in data-sheet, with ADJ pin floating the expected voltage on ADJ pin should be about 1.2V. Now connecting ADJ pin with 1K resistor followed by supply voltage, as ADJ pin supply voltage increases beyond 1.2V then VOUT will also increase above 2.5V. This suggest that increasing ADJ pin supply voltage will inject or LT1009 ADJ pin begin to sink current which will pull VOUT high.

    To understand the 2nd plot you shared where setting ADJ pin supply voltage to 0V and VOUT pulled to 5V, can you please share more details about your test setup? Was this measurement conducted on bench? Test schematic would be helpful.

    Regards,

    Anshul      

  • Hi Anshul,

    Many thanks for your support. Sorry for my late reply.

    I understood the LT1009 can work like a comparator when it is in open loop, working somehow like a TL431. This behaviour is what I was expecting but not the one I got from my simulation. That's the purpose of my question here.

    The second plot is the one I was expecting to have keeping the same schematic as in the first image, no change in schematic.

    Kind Regards,

    Jose

  • Hi Jose,

    Looking at LT1009 schematic diagram within data-sheet, ADJ pin already incorporates closed loop configuration. Due to this LT1009 cannot work like a comparator. On the other hand, TL431 is open loop and can operate as a comparator.

    Regards,

    Anshul

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