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LTC3623 - Simulation for negative output

Dear support team:

I'm trying to simulate the circuit from the figure 10 of the LTC3623 datasheet, as I need a negative output. Please refer to the following schematic:

After running the simulation, the SW node begins to oscillate and the output voltage starts to go under 0 V. At some point the inductor current increases indefinitely and it just stops working:

Can you please help me to figure out what's happening? We just bought the development kit and I would like to try the IC in this configuration.

Thanks in advance!

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  • Thanks Paul, it seems to be working now. Can you please help me check the following?

    1) In the same circuit, if I try to use an 80k feedback resistor (for setting the output voltage to -4 V), the output voltage can't go below -2.38 V and there is a high ripple current on the inductor (6 A peak to peak).

    2) I need to set up the output voltage with an external source with an added offset of 0.3 V. In the positive configuration it is easy as I can put the required voltage directly into the ISET pin, but for the negative configuration I can't figure it out. What I did is the following:

    Instead of connecting VOUT directly to GND, I'm putting the required voltage here. The 0.3 V offset is created by the 6 k resistor on ISET pin.

    This configuration works but it has the same limitation of the point 1 (can't go below -2.38V) and it's creating a high inrush peak on the inductor:

    Can you please confirm if this configuration can be used? Is there any way to configure the output voltage by using a positive supply instead of negative? As I don't want to add a secondary negative supply on the circuit.

    Thanks again for your help!

  • Hi Ody,

    This kind of question must be posted under Power by Linear Forum as that forum is being maintained by people more equipped with application related concern. This forum is for LTspice and/or simulations issue related questions only.

    However since you already raise it here, I might as well provide you a solution.

    Please read the datasheet carefully. This IC has several loops working together to generate/limit/control the output voltage.

    you mentioned that the voltage clamps to -2.38V. this is because, the IMON pin is connected to GND.

    Since you are operating at negative output, the PGND pin is connected to the negative output which is also the reference of all internal circuitry except those mentioned referenced to GSNS pin. with this scenario, when the output is equal to -2.35V, IMON sees it at +2.35V. thus limiting further reduction of voltage.

    You can disable this feature by connecting it to the PGND instead. As much as the simulation is concern since this is the reference.

    However, I am not sure if the actual part will work that way.

    Please be careful on you testing. Happy simulation.