Can I use the LT3092 with negative voltages?

To add on, if you can use it with negative voltages and negative linear regulators, where should it be placed in series: before the input of the LM337 as shown in my example picture or after the output of the LM337 (i.e., in series after C3)?

I don't think you are looking at this the correct way. 

Please take a look at the simulation attached.

LT3092_Negative.asc

Although the LT3092 might operate in a negative supply at I show in the simulation,

The voltage that the IN pin will not be sitting at -35V as you are referenced to ground and your resistive load and load current will determine where the IN pin voltage.  

Let me know what you think after looking at the Ltspice simulation.

Charly

charlyelkhoury I think I see what you're saying. I thought the LT3092 would limit the current like how a voltage regulator regulates the voltage. In your example, with a max voltage of 35 volts and LT3092 limit to 1 mA, when the load is less than 1 mA (say, 350k ohms), I don't see 35 volts, but a little less than that at 34.7 volts. At 35k ohms (1 mA), I see 33 volts. In the attached example schematic, I see the same thing when I pair it with a LT3015 linear regulator. It's set for -30 volts while the LT3092 is set for 2 mA. Despite all that, when the load draws less than 1 mA, I expected the voltage across the load to be -30 volts, but it's not as a result of the load and the current it draws.

Sorry, I guess this won't have the desired effect after all. I don't want my load voltage to be controlled by the load current. I was looking for a negative linear regulator with current limiting capabilities down to 1 mA. I thought the LT3092 would work in this case.

PS: Clearly, it won't work for my purposes here, but in general, can the LT3092 be used as a current source involving negative voltages?

LT3092_Negative_2_Flow.asc

I think you conclusion is correct; this will not act the way you want it to for this application.

As far as will this part work in negative environment like we used it in LTspice, the answer is yes, but for specific applications.

As long as we don't violate the parts limits it will be fine.

Charly

charlyelkhoury I apologize, but I was thinking about it again, and I think there was a factor in the previous simulations that may change things. In your simulations, the test load was a resistor, meant to represent a constant current draw. In that sense, while the LT3092 limited the current to whatever it was set to, the voltage as seen by the load was determined by how much current it can draw. That I get, but what if the load was a variable load like a photodiode? Can the LT3092 act as a current limiter in this case?

Take this schematic I've posted below. The LT3015 generates the negative voltage that I want, but it has no current control. So, the LT3092 is supposed to act as the current regulator in this case. The load here is a variable load, set to trigger at 120 ms. It's supposed to act as stand-in for a photodiode, drawing 1 mA when active and no current when not active. In this case, while the LT3015 maintains the voltage seen across the load, the LT3092 limits the current. You can see the voltage change when the load is active and drawing current, but when it's inactive and not drawing current, the voltage across the load stays the same as set by the LT3015. In that sense, would the LT3092 work in this case compared to a resistive load?

LT3092_Negative_4.asc

the way you did it in this latest simulation is better, as your negative LDO is setting the negative voltage you need, and you are only using the LT3092 to limit the current, 

If this looks good for your application, I would try it with the eval board just to make sure the voltage regulation is acceptable for your application before building your own board.

Charly.