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Products Mentioned

LT8641 Recommended for New Designs
The LT8641 step-down regulator features Silent Switcher architecture designed to minimize EMI/EMC emissions while delivering high efficiency at frequencies...
Datasheet
LT8641 on Analog.com

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LT8641 not starting properly stays at 1.6V Vout

		
	
				

			

									
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				                on Jul 16, 2026 
				
			
			
		

		
		                     
				
        		

		    
		    		    

		      		            		        
Category: Hardware

    		        		            		    
Product Number:  LT8641  

    		        		    		    

		    		    
		    			

			    
Hello,


We are using the LT8641 on our PCB, and it works quite well in general. However, from time to time, we experience problems with a few units. In these cases, the IC does not start up properly, and VOUT remains at 1.6 V.


There is no short circuit and no increased power consumption on the 5 V side. It seems as if the IC tries to start up but then stops and regulates the output voltage to 1.6 V without switching. Same behavior in bust mode and spread spectrum mode.


Could you please advise what could cause this behavior or which parameters we should check?


The IC is configured according to the recommended application circuit in the datasheet:
 


SW pin:
 


IC: LT8641
VIN: 24V
VOUT: 1.6V (Soll: 5V)
FB: 0.264V
FB divider: 1M / 191k
EN: 24V
SS/TRK: 2.47V
INTVCC: 3.23V
BST-SW: 2.7V
RT: 41.2k to GND, RT pin ~1.1V
SYNC/MODE: INTVCC via 10k (Spread Spectrum)
SW: initial burst pulses, then no switching




Thank you in advance for your support.

			

			
									
		                             
                        

    

                                            
                 hardware
                
                                                    
                 switching regulators
                
                                                    
                 LT8641
                
                                                    
                 Step-Down (Buck) Regulators
                
                                
            

    
    
		


				
				        
        		

			
						
									
												
			
			
                        
                                    
                        
                                                    
                        
            
			            
						
						
			
			
			
			
		


        
        
        
    
					

			





















	

	

					



			


				
	









	
    
	

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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Jul 20, 2026 2:38 PM
            
            
						
			            		
    
		
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    Hi,
     
    Where did you purchase the LT8641 chips from? Can you please provide us with a purchase order?
     
    In addition, how many boards have you tested and how many have failed? Does replacing the IC on the failing boards fix the problem?
     
    Finally, do you have an LTSpice model and LTPowerCAD file for this design that you could share? At first glance, we did not see any issues with your schematic. Also, have you tested your design on the demo board?
     
    Thanks!
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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                on Jul 21, 2026 11:23 AM
            
            
						
			            		
    
		
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    Hi karmijo327,
    

    We purchase the LT8641 through authorized distributors.
    

    We are seeing a failure rate of approximately 1–2%. Replacing the IC resolves the issue.
    

    At the moment, we do not have a SPICE model or an evaluation board available for further investigation.
    

    Is this behavior a known issue?
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Jul 22, 2026 2:17 PM
            
            
						
			            		
    
		
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    Hi,
     
    With a low failure rate, purchasing from authorized distributors, and the fact that replacing the chip resolves the issue, your board is likely having issues due to problems during assembly and soldering. Did you have a third party assemble the boards?
     
    Thanks,
     
    -Kenneth Armijo
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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                on Jul 23, 2026 2:44 AM
            
            
						
			            		
    
		
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Thank you very much for your assessment. We also thought about issues due to problems during assembly and soldering.
    Do you have any recommendations regarding the reflow soldering temperature profile and the recommended stencil thickness?
    

    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Jul 24, 2026 10:23 AM
            
            
						
			            		
    
		
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    Hi,
     
    For stencil thickness, we would recommend 5mil thick. For the solder reflow profile, we would recommend following the standard Pb-free JEDEC J-STD-020 process for QFN packages: SURFACE MOUNT PRODUCTS. 
     
    Thanks!
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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