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LT3042 PG pin when not powered (IN = 0 V)



		

			

									
					    dnintegrolution
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Oct 5, 2022 
				
			
			
		
    

				
		
    
		    		    
		    
		    		            		        
    Category: Hardware
    
    		        		            		    
    Product Number:  LT3042 
    
    		        		    		    	
			
    Does the PG pin of the LT3042 sink current when the device is not powered (IN = EN = PGFB = GND = 0 V)?
    

    The Absolute Maximum Ratings seem to allow this condition (maximum PG pin voltage is 22 V without regard to IN voltage).
    

    The LT3042 model in LTspice suggests PG does not sink current until IN reaches about 1.22 V (maybe the Vgs threshold of the PG output transistor?) I am not sure if this behavior is modeled realistically in LTspice.
    

    Motivation: I want to pull up PG and connect it to an N-MOSFET gate to turn on a relay (driven by a separate power rail). This rail can be high when the LT3042 supply is off. Thus if PG is high-impedance, the relay would wrongly close. (In this case I would need another transistor to break the relay circuit when IN is low.)
    

    Thank you!
    
								
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	















    
	
    
	
    
					



			








	
	
	
	



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													+1
							
			
				
									
							        
			
			
			
      •  Analog Employees  
      
            
			                            
                        
                on Oct 6, 2022 2:04 AM
            
            
					
      
	
      

		
		
      
					
      The LT3042 PG pin is not going to pull down until the LT3042 is enabled with VIN above its UVLO.  I show a scope capture below for the LT3045, but the LT3042 will behave the same way.  Also, I usually ask customers to only load PG to 100uA - 200uA which is similar to the condition of testing PG.  I show the line item from the datasheet Electrical Characteristics table that has that condition in the other image below.  In other words, your resistor that pulls up the gate of your FET will be so large that your FET will only turn on slowly and that may stress your FET.
      

      

      

      LT3042 PG current_2.png
      

      

      Conditions:  LT3045EDD, DC2491A, 5VIN, 3.3VOUT at 100mA, VCIN VEN VPG VOUT, EN startup_0.bmp  PG is pulled up to VIN)  PG IS MISLABELED OUT.
      

      LT3045EDD, DC2491A, 5VIN, 3.3VOUT at 100mA, VCIN VEN VPG VOUT, EN startup_0.bmp
      

      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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													+1
							
			
				
									
							        
			
			
			
        
            
			                            
                        
                on Oct 6, 2022 3:01 PM
            
            
												
						in reply to ARad
					
									
        
	
        

		
		
        
					
        Hi Noel and ARad,
        

        Thank you for your answer, this gives me the information I needed.
        

        The project is an early stage prototype and it's unclear when it will enter production. Initial volume would be low: probably only 100 boards, each board using one LT3042 and one LT3093. I don't require any further support at this time but I thank you for the offer to check in with an FAE.
        
			
        

		
		
		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
				
						
						
				
				
		
		
        
			
		
        
	
        

        

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													+1
							
			
				
									
							        
			
			
			
          
            
			                            
                        
                on Oct 6, 2022 3:01 PM
            
            
												
						in reply to ARad
					
									
          
	
          

		
		
          
					
          Hi Noel and ARad,
          

          Thank you for your answer, this gives me the information I needed.
          

          The project is an early stage prototype and it's unclear when it will enter production. Initial volume would be low: probably only 100 boards, each board using one LT3042 and one LT3093. I don't require any further support at this time but I thank you for the offer to check in with an FAE.
          
			
          

		
		
		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
				
						
						
				
				
		
		
          
			
		
          
	
          

          

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