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LT3797 Production
The LT3797 is a triple output DC/DC controller designed to drive three strings of LEDs. The fixed frequency, current mode architecture results in stable...
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LT3797 on Analog.com

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LT3797

		
	
				

			

									
					    edg10
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Feb 17, 2021 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
		    			
    
			    
    Is there a way to decrease the LED start-up time of the first PWM pulse? Repeating PWM pulses switch the LEDs on within roughly 200ns. The very first PWM pulse however takes upwards of 30us to reach full power to the LEDs. The delay seems to occur if the PWM pulse is inactive for more than about 2-3 seconds. The images below are from a DC1784B demo board (LT3797) with 24V input driving a string of 12 LEDs.
    

      
    
			
    
			
									
		                             
                        

    
    
                                            
                 Step-Down (Buck) LED Drivers
                
                                                    
                 LT3797
                
                                                    
                 LED Driver IC
                
                                
            
    
    
    
		
    

				
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                                                    
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
      
	

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Feb 18, 2021 12:22 PM
            
            
						
			            		
      
		
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      Hello,
      

      up to my knowledge the problem comes from the output cap. The energy from the first pulse is shared between charging the cap and current for the LED. For longer PWM=low times the cap discharges again and you run into the same problem again with your first pulse as the cap needs energy as well.
      

      

      kind regards
      

      Markus
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
				
				
				
		
		
      
			
		
      
	
      

      

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                on Feb 18, 2021 1:33 PM
            
            
												
						in reply to MarkusHP
					
										
			            		
        
		
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        Thanks for the response MarkusHP. I take it the output caps you refer to are the caps placed on the OUT side of the boost driver connected to ISPx? The DC1784B demo board has 3x 4.7uf caps on these OUT lines. Also, when the cap discharges between long PWM pulses, this results in 0V on the Vdrive signal in my first graph above? I noticed with the repeated PWM pulses that the Vdrive voltage starts around 25V and ramps quickly to the final drive voltage as opposed to starting at 0 and ramping up over 30-40us. Is there any way to make the controller hold the 25V level and not ramp down to 0V between PWM pulses?
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
				
				
				
		
		
        
			
		
        
	
        

        

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