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LTC4421 Recommended for New Designs
The LTC4421 connects one of two input supplies to a common output based on user-defined priority and validity. By definition, the supply connected to V1...
Datasheet
LTC4421 on Analog.com

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Primary input turns off secondary input

		
	
				

			

									
					    rcooke
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Sep 5, 2026 
				
			
			
		

		
		                     
				
        		

		    
		    		    

		      		            		        
Category: Hardware

    		        		            		    
Product Number:  LTC4421  

    		        		    		    

		    		    
		    			

			    
Hi All,


I have a weird situation with the LTC4421 chip. I have used it in the past without any issues but with this board it is misbehaving badly. I have built 10 boards with the current design without any issues but this 11th board isn't working.


The problem is If I have just the secondary power input connected the /VALID2 operates correctly but as soon as I connect the primary supply both /VALID1 and /VALID2 go high even with both input voltages within the proper ranges. I used a scope to check to see if there was any oscillations but the inputs are steady. The OV, UVF, UVR inputs are steady.


I have checked that the resistor values are correct. I used the excel tool to calculate the resistor values.


I am at a loss to explain what is happening and why this board isn't working as it should. Here is the schematic:


 


Thanks,


Richard

			

			
									
		                             
                        

    

                                            
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                on Sep 5, 2026 4:51 PM
            
            
						
			            		
    
		
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    I need to add that the /VALIDx pins are connected to a NUcleo-L031 board with the inputs set as pull-ups to 3.3 V. 
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Sep 11, 2026 9:29 AM
            
            
						
			            		
    
		
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    If other 10 boards are working, I would recommend comparing the failing board directly against one of the working units.
     
    Could you try these two checks?
    Temporarily disconnect /VALID1 and /VALID2 from Nucleo, pull them up locally to 3.3 V, and repeat the same test where V2 is present first and V1 is then connected. Let me know if both signals still go high.
     
    On one working board and the failing board could you compare /VALID1, /VALID2, SHDN, and INTVCC as V1 is connected? 
    Also when it fails, do both /VALID signals remain high or do they eventually return low?
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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                on Sep 11, 2026 6:53 PM
            
            
						
			            		
    
		
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    Unfortunately I no longer have the working boards. I've shipped them to the customer. All I have assembled are the 2 that are showing this crazing behavior.
    

    I put a scope on /VALID2B and it looks like it tries to go to zero every 53ms. It falls for 128us. /VALID1B stays constant at 3.25V.This is with the Nucleo-L031 inputs set as pull-ups.
    

    I changed the inputs to to pull-ups and used 10K resistors from /VALID1B and /VALID2B to the INTVcc pin. If I connect only the IntBatt the signal switches correctly from 3.7V to 0.0V and /VALID2B is OK. As soon as I turn on the Primary input (13.8V) the /VALID2B signal does the same thing as it did with the Nucleo pull-ups. It stays high and goes low every 53ms but the pulse width is a bit wider at 1.24ms.
    

    SHDN = 3.3V. Does not change.
    

    INTVcc = 3.785V. Every 53ms it falls to 3.52V for 112us.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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