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LT317A Loop stability design

		
	
				

			

									
					    UHR220
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Jul 8, 2025 
				
			
			
		

		
		                    	    

        	    
Thread Summary

                The user asked how to simulate the Bode plot for a circuit similar to the one in the LT317A datasheet to find the phase and gain margin using LTspice. The final answer confirmed that the circuit worked well with the .fra component placed in series with the upper sense resistor, but adding a 10uF capacitor in parallel with the lower sense resistor prevented the 0dB crossover from being found. The accompanying answer provided a circuit diagram and an LTspice file for reference.
                        	    

                
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Category: Software

    		        		            		    
Product Number:  LT317A  

    		        		        		        
Software Version: LTspiceXVII and/or LTspice24

    		    		    

		    		    
		    			

			    
How can I simulate the Bode plot for this device(circuit) to find the Phase and gain margin using LTspice software?


My circuit looks similar to the onces shown in the datasheet.


Input voltage: 12V (9V to 12V)


Output voltage: 5V


Output current: 150mA to 1A


Regards


Udo

			

			
									
		                             
                        

    

                                            
                 software
                
                                                    
                 LT317A
                
                                                    
                 LTspiceXVII and/or LTspice24
                
                                                    
                 LDO Linear Regulators
                
                                                    
                 Positive Linear Regulators (LDO)
                
                                
            

    
    
		


				
				        
        		

			
						
									
												
			
			
                        
                                    
                        
                                                    
                        
            
			            
						
						
			
			
			
			
		


        
        
        
    
					

			





















	

	

					



			


				
	









	
    
	

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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Jul 8, 2025 4:23 PM
            
            
						
			            		
    
		
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    It is easiest to use the FRA tool in LTspice, but if you have a physical Bode plotter, I would just put the injection resistor in the same place as the FRA tool.  The injection resistor can even be ground referenced.   
    

    

    sim ckt and result_1.png
    

    

    LT317A_5.asc
    

    

    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
				
						
				
				
				
		
		
    
			
		
    
	
    

    

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                on Jul 9, 2025 3:45 AM
            
            
												
						in reply to ARad
					
										
			            		
    
		
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    LT317A_LTspiceXVII_Udo.ascThank you very much for your file which I tried to simulate. I'm surprised where you placed the .fra component. Traditionally a stimulator is placed in series to the upper sense resistor. Nevertheless, your circuit worked well, provided the lower sense resistor has no paralled capacitor. In the LT317A datasheet, very often a 10uF is connected in parallel to the lower sense resistor. If I do that in your circuit, the 0dB crossover is not found.
    

    Meanwhile I've used LTspiceXVII as well, and the acsweep was placed in series to the output.
    

    I would like to send you this file for inspection, but I don't know how to attach this file.
    

    Best Regards
    

    Udo
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
						
				
				
				
		
		
    
			
		
    
	
    

    

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