voltage regulator lm3940

[sumit garg]

i did everything but i am not getting any output

 

[betwixt]

It should work. Is this just in simulation or is it a real circuit?

Brian.

 

[sumit garg]

its just in simulation for now

 

[betwixt]

Something is wrong with the simulation settings or the device model.
Try adding a resistor (1K) across C4 to force some load on the regulator and see what happens but it isn't normally needed.

Brian.

 

[sumit garg]

I added the resistor then too nothing happened but when i removed ground terminal it is showing like this:

but it should give me output of 3.3 V bcz i m using (5v to 3.3 v)LDO regulator, i dont know what is happening

 

[stenzer]

Hi,

I'm a little bit confused by the part number of the linear regulator, as it indicates a 3 V output voltage (the .3 at the end is missing). Where does this spice model come from, is it a built in one?

I don't know how accurate the regulator is modeled, but you might test the simulation with a resistor of ~0.5 Ohm in series with C4, see section 8.2.2.1.2 ESR Limits on page 9 in [1].

[1] https://www.ti.com/lit/ds/symlink/l...l=https%3A%2F%2Fwww.ti.com%2Fproduct%2FLM3940

BR

 

[betwixt]

I think if the resistance in series with R4 isn't fitted it only affects the stability and maybe transient response, there should still be some voltage there. My guess is the model for U5 is incorrect or it is a renamed copy of a higher voltage regulator that needs >5V at its input..

Brian.

 

[danadakk]

There is no specific min load requirement but datasheet output V specs are
stipulated for >= 5 mA which is implied min load. Also the stability < 170 mA
loads has issues below that current spec in this graph from datasheet,
whether or not sim cares about this not sure, depends on model completeness :

Regards, Dana.

 

[d123]

Hi,

See if the LM3940 model in the programme you are using to simulate with has a netlist - if it does, post it here and maybe one of the experts can see what is wrong with the model/simulation set-up.

Did you assign the in, gnd, out pins to the model? (Try swapping in with out, etc.)

Sometimes an AC simulation works where a DC simulation doesn't (but should) or a transient analysis might show Vreg working, or at least show what start-up waveforms look like and give some clues.

 

[stenzer]

Hi,

is there even a 3 V version of the LM3940 available? I checked several distributers, I couldn't spot a 3 V version.

BR

 

[sumit garg]

Hi,

is there even a 3 V version of the LM3940 available? I checked several distributers, I couldn't spot a 3 V version.

BR

no, actually this is also a 3.3V version .

--- Updated Jul 28, 2022 ---

Hi,

I'm a little bit confused by the part number of the linear regulator, as it indicates a 3 V output voltage (the .3 at the end is missing). Where does this spice model come from, is it a built in one?

I don't know how accurate the regulator is modeled, but you might test the simulation with a resistor of ~0.5 Ohm in series with C4, see section 8.2.2.1.2 ESR Limits on page 9 in [1].

[1] https://www.ti.com/lit/ds/symlink/lm3940.pdf?ts=1658856218089&ref_url=https%3A%2F%2Fwww.ti.com%2Fproduct%2FLM3940

BR

--- Updated Jul 28, 2022 ---

Hi,

See if the LM3940 model in the programme you are using to simulate with has a netlist - if it does, post it here and maybe one of the experts can see what is wrong with the model/simulation set-up.

Did you assign the in, gnd, out pins to the model? (Try swapping in with out, etc.)

Sometimes an AC simulation works where a DC simulation doesn't (but should) or a transient analysis might show Vreg working, or at least show what start-up waveforms look like and give some clues.

Netlist

ISIS SCHEMATIC DESCRIPTION FORMAT 8.0
=====================================
Design:   voltage regulator.pdsprj
Doc. no.: <NONE>
Revision: <NONE>
Author:   <NONE>
Created:  28-07-2022
Modified: 28-07-2022

*PROPERTIES,0   

*MODELDEFS,0   

*PARTLIST,6   
C3,AVX0603Y5V100N,0.47u,CODE="Farnell 317-287",EID=17,PACKAGE=0603_CAP
C4,AVX0603Y5V100N,33u,CODE="Farnell 317-287",EID=18,PACKAGE=0603_CAP
D1,LED-RED,LED-RED,BV=4V,DRIVE=1,EID=1,IMAX=10mA,PACKAGE=LED,ROFF=100k,RS=3,STATE=0,TLITMIN=10m,VF=2.2V
R1,9C04021A3000FLHF3,1k,CODE="Digikey 311-300LCT-ND",EID=6,PACKAGE=RESC1005X40,PRIMTYPE=RESISTOR
U1,LM3940IT-3,LM3940IT-3,EID=5,PACKAGE=TO254P470X1028X1955-3P
V1,VSOURCE,5V,EID=3

*NETLIST,4   
#00002,3
C3,PS,1
U1,PS,1
V1,PS,1

#00003,2
C4,PS,1
R1,PS,1

#00004,3
D1,PS,A
U1,PS,3
R1,PS,2

GND,6,CLASS=POWER
GND,PR
C3,PS,2
V1,PS,2
U1,PS,2
C4,PS,2
D1,PS,K

 

[d123]

Hi,

Misunderstanding , I meant the netlist for the LM3940 model. It might also be called 'view macro' or suchlike. Maybe right-clicking on it brings up a menu with that option in the simulation tool.

 

[sumit garg]

Hi,

Misunderstanding , I meant the netlist for the LM3940 model. It might also be called 'view macro' or suchlike. Maybe right-clicking on it brings up a menu with that option in the simulation tool.

no, i cant see anything what you are saying
these are the options that shown on right clicking model

 

[d123]

'Edit properties', maybe? I'm not familar with that sim tool.

I don't know where the model came from, but it might be worth looking for another model and trying that. Maybe you downloaded a PSpice model from the manufacturer?

By now, saying where the model is from and how it was installed (if it's not already in the simulation program), might help.

 

[stenzer]

Hi,

which Spice program are you using? Looks like Proteus to me.

Where does this spice model come from, is it a built in one?

I couldn't find a SPICE model provided by the manufacturer (TI). If it is Proteus, I could not spot it within the pretty old (2003) library list [1]. The LM3940 is available at least scince 1999.

Check the program folder on your harddrive if you can spot a folder named library, models ore something like this. Have a look if there is a file named LM3940(xxx). Open it with a text editor and share its content.

[1] https://www.ele.uva.es/~jesman/BigS...Software-Manuales/EBook - Proteus Library.pdf

BR