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LTC2864 Production
The LTC2862/LTC2863/LTC2864/LTC2865 are low power, 20Mbps or 250kbps RS485/RS422 transceivers operating on 3V to 5.5V supplies that feature ±60V overvoltage...
Datasheet
LTC2864 on Analog.com

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LTC2864 and ESD



		

			

									
					    MikeMNK
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on May 13, 2024 
				
			
			
		
    

				
		
    
		    		    
		    
		    		      		            		        
    Category: Hardware
    
    		        		            		    
    Product Number:  LTC2864  
    
    		        		    		    	
			
    We are using the LTC2864-2 in an existing design and it's behaving as expected.
    

    

    We have a new design of the system, which opens the Interface I/O side of the design to ESD problems. We're trying to meet IEC61000-4-2 Level 4 of +/-15kV.
    

    We've added a number of TVS diodes around the system, trying to harden it. We have a number of LDO regulators and a Microcontroller. After performing 1-2 x 15kV ESD strikes, we notice that the LTC2864-2 we're using is the damaged part. The strange part is that we're seeing a short to ground through one of the RS485 exposed pins (pin 6 and pin 7 of the LTC2864-2), which is the Z and Y Driver Output pins on the device. 
    

    I would have expected these to be protected up to +/15kV ESD as per the datasheet. However, I suspect the issue may be the Driver Enable pin that is actually get hit, which in turn is damaging the internal circuits and shorting the Driver Output pins to ground.
    

    

    I hope you can help me on this.
    

    

    Thanks in advance,
    

    Best regards,
    

    Mike O'Keeffe
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 hardware
                
                                                    
                 RS-232 / RS-422 / RS-485 / RS-562
                
                                                    
                 ltc2864
                
                                                    
                 RS-485 and RS-422
                
                                                    
                 esd
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
      
	

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on May 13, 2024 11:14 PM
            
            
					
      
	
      

		
		
      
					
      Hi Mike,
      

      The +/-15kV ESD spec on the LTC2864 datasheet is for Human Body Model (HBM), not for IEC 61000-4-2. They are two different test models, and the IEC 61000-4-2 strike current is higher. Please see below datasheet spec example. 
      

       
      

      

      Best,
      

      Jane
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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                on May 14, 2024 4:25 AM
            
            
												
						in reply to YaoJ
					
									
        
	
        

		
		
        
					
        Hi Jane,
        

        Thanks for your reply. I can't find this table in the LTC2864 datasheet. I have revision C 03/14 directly from your website. 
        

        This still doesn't solve my problem. I need to harden our device to meet +/-15kV and possibly even further. As I mentioned I'm striking on the side of RO,RE,DE and DI trying to see what are the weak points. During my testing I found that the Z,Y Outputs are the ones that are getting damaged, which I'm surprised by, as I'm not striking on that side of the device. I was expecting RO,RE,DE and DI to get damaged. 
        

        

        Can you suggest a method to protect the device from both sides or should I just add TVS diodes, current limiting resistors to every pin?
        

        

        Regards,
        

        Mike
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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        •  Analog Employees  
        
            
						
                            
            
            				
            
                on May 16, 2024 11:05 AM
            
            
												
						in reply to MikeMNK
					
									
        
	
        

		
		
        
					
        Hi Mike
        

        The above table is from a different datasheet. I just want to show you if a part guarantees any IEC 61000 specs, the datasheet should show with details. Please see below figures regarding the HBM and IEC61000-4-2:
        

         
        

         
        

        Please be sure you can strike on the correct pin. You can damage the part or even other parts on your board or system. You can add a short wire on top of a pin that you need to strike on for Air Discharge test.
        

        The LTC2864 doesn’t guarantee any IEC 61000 spec. To achieve the IEC 61000-4-2 level 4 (+/15kV Air Discharge and +/-8kV Contact Discharge) with LTC2864, external protections are needed. TVS diodes on the bus lines will help to achieve level 4. Current resistors will help on the RE, DE, RO, and DI pins. It is challenged to find TVS clamps at low voltage, like 6V. We have boards with SMCJ33CA on bus lines achieved level 4.  You should always test and verify TVS diodes in your application since it could impact data rate, and it is also layout and system dependent.
        

        Best,
        

        Jane
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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