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LTC4357 Recommended for New Designs
The LTC4357 is a positive high voltage ideal diode controller that drives an external N-channel MOSFET to replace a Schottky diode. When used in diode...
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LTC4357 on Analog.com

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LTC4357 - OUT Voltage is high, IN voltage is 0, GND pin is floating - will this safely turn off the ORING?



		

			

									
					    HLevy585
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jan 17, 2023 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    ORING connected to DC-DC on the source and a battery bank on the drain. when the controller senses a positive voltage difference between the IN and OUT pins the controller turns on the MOSFETs.
    

    Lets say I now want to turn off the controller. by the datasheet it will turn off when it senses a negative voltage of 25mV which is not good at all because it will never turn off on its own unless high reverse current will flow back into the ORING.
    

    

    The datasheet suggest to disconnect the ground pin to turn off the controller but in this situation the output voltage is held high by the battery bank while the input voltage can drop down to 0.
    

    The following picture dipicts the situation - Vout is big capacitor bank held at 58.5V.
    

    ORING_IN is the DCDC output and can be shut down to 0V.
    

    ORING_ON is now off and so the GND pin is floating.
    

    will this safely turn off the ORING and the controller will remain undamaged?
    

    

     
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 LTC4357
                
                                                    
                 Power Monitor  Control  and Protection
                
                                                    
                 PowerPath  Ideal Diodes  and Load Switches
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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      •  Analog Employees  
      •  Super User  
      
            
						
                            
            
            				
            
                on Jan 17, 2023 8:58 PM
            
            
					
      
	
      

		
		
      
					
      Hi Levy. 
      My answers below:
      

      HLevy585 said:
      Vout is big capacitor bank held at 58.5V
      

      Be aware that ideal diodes can't control inrush into COUT.
      

      HLevy585 said:
      will this safely turn off the ORING and the controller will remain undamaged?
      

      If the powerpath was on and you suddenly disconnected ground, there is no guarantee what voltage GATE would be, since the part is floating.
      You need to add a 10Meg resistor from IN to GATE so VGS will settle to VGS=0V once the part is floating.
      

      You will not be able to use this "shutdown" FET to select which power supply you want to pass to the shared output (you need a prioritizer for that).
      The body diode will provide a path from IN to OUT, regardless if VGS=0V.
      

      I don't think the part would be damaged.
      

      Extra:
      R81 isn't necessary since Q27 is a "diode" FET, so no risk of FET self-oscillation.

      

      Hope this helps,
      -Aaron
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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													0
							
			
				
									
							        
			
			
			
      •  Analog Employees  
      •  Super User  
      
            
						
                            
            
            				
            
                on Jan 17, 2023 8:58 PM
            
            
					
      
	
      

		
		
      
					
      Hi Levy. 
      My answers below:
      

      HLevy585 said:
      Vout is big capacitor bank held at 58.5V
      

      Be aware that ideal diodes can't control inrush into COUT.
      

      HLevy585 said:
      will this safely turn off the ORING and the controller will remain undamaged?
      

      If the powerpath was on and you suddenly disconnected ground, there is no guarantee what voltage GATE would be, since the part is floating.
      You need to add a 10Meg resistor from IN to GATE so VGS will settle to VGS=0V once the part is floating.
      

      You will not be able to use this "shutdown" FET to select which power supply you want to pass to the shared output (you need a prioritizer for that).
      The body diode will provide a path from IN to OUT, regardless if VGS=0V.
      

      I don't think the part would be damaged.
      

      Extra:
      R81 isn't necessary since Q27 is a "diode" FET, so no risk of FET self-oscillation.

      

      Hope this helps,
      -Aaron
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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