I have a quick question regarding LT3091. This is a negative voltage regulator with programable current limit but with a fixed output voltage right ? The low dropout voltage is 300 mV. The output is rail-to-rail.

The output voltage has to be Vin - 0.3 V, right ?

Vin = -5 V
Vout = -4.7 V ?

https://www.analog.com/en/products/lt3091.html

 

Read the datasheet! You can set the output voltage by varying the resistor at pin "SET" between 0 V and -32 V. Your input voltage has to be at least 0.3 V lower than the output voltage. This is the minimum dropout voltage. If your input voltage is lower, no problem. The regulator will always convert the product

|V_in-V_out|*I_out

into heat.

 

Read the datasheet! You can set the output voltage by varying the resistor at pin "SET" between 0 V and -32 V. Your input voltage has to be at least 0.3 V lower than the output voltage. This is the minimum dropout voltage. If your input voltage is lower, no problem. The regulator will always convert the product

|V_in-V_out|*I_out

into heat.

Thanks. I got it.

 

Below is the LTspice sim of the LT3091 with four different values for the voltage set resistor R1 (giving 2.5V, 5V, 7.5V, and 7.66V ), while increasing the current limit resistor R2 and reducing Rload to show an increase in current-limit with time (curved lines going from 0.4A to 1.25A limit):
The 7.66V output is limited by the dropout voltage of about 336mV for the Spice LT3091 model

Edit: Increased step voltage range to show dropout voltage.

 

Dropout can be as high as -525mV.
Min input voltage is -1.5v (but can be as high as -1.9v)
Min output voltage is Vin+Dropout = -1.9v + -0.525v = -2.425v to maintain regulation.

 

Min output voltage is Vin+Dropout = -1.9v + -0.525v = -2.425v

|Vin| < |Vout|?

 

I have looked at the datasheet again and understand that the resistor Rlim between pin ILIM and GND will set the output current limit.

The resistor Rset between pin SET and GND adjust the output voltage according to Table 1 of the datasheet.

 

I am wondering if there exist any positive linear voltage regulator similar to LT3091 with adjustable current limit and adjustable output voltage ?

I know there exist several positive adjustable voltage regulators. Usually there are two external resistors that can set the output voltage level. How current limit can be added to such voltage regulators ?

 

positive linear voltage regulator similar to LT3091

LT3081.

 

Just another question.
Is that possible to use digital controlled potentiometer operated by microcontroller or FPGA and use instead of fixed resistors at SET and ILIM pins to make it digitally controlled adjustable voltage linear regulator ?

 

Is that possible to use digital controlled potentiometer

Only if the voltage across the digipot does not exceed its supply voltage.

 

Only if the voltage across the digipot does not exceed its supply voltage.

Yes, the voltage on the pot wiper would equal the output voltage, so the pot supply would need to be equal to the highest output voltage.
The input voltage could be used if the pot could tolerate that voltage (many are limited to 5.5V).

Here's an example that goes to 36V.
But note that it has a ±20% initial resistance tolerance, and just 8-bits of resolution.
Also note the polarity of the required digital control voltages.

 

Just another question.
Is that possible to use digital controlled potentiometer operated by microcontroller or FPGA and use instead of fixed resistors at SET and ILIM pins to make it digitally controlled adjustable voltage linear regulator ?

This could be possible but you have to keep two things in mind:
1. The absolute value of a digi-pot can vary up to 20% (again: read the according datasheet) This tolerance will also be the tolerance of your output voltage.
2. This might not work with negative voltages unless you do a level shift maybe with optocouplers.

 

Here's a simple way to control the output voltage with a positive control voltage (Cntl) using an opto coupler:

But note that the output vs. input voltage is affected by the transfer ratio of the opto.
This could be essentially eliminated, along with the 1V input offset, by using a linear type opto that has two matched outputs (e.g. IL300) with feedback from one output to an op amp driver.