Question about an external PMW dimming for LT3922

Question

Hi, 
 I have some questions about duty signalof external PWM signal to turn LEDs off? 
 Please take a look at the attachment. 
 When my external PWM duty cycle is set to 1020/1024 (D=99.6%), my LEDs are not turned off completely. 
 When my external PWM duty cycle is set to 1023/1024 (D=99.9%), my LEDs are turned off. 
 Question 
 1. What is the possible reason about the difference in those results? (D=99.6% and =99.9% seem to be almost same.) 
 2. Toff (PWM duty signal OFF time) and Ts (switching period) are related to turning LEDs off? 
 Considering the toff of each duty cycle, D=99.6% has 3.9us, which comprise app 7.8 times as long as Ts=0.5us. 
 On the other hand, D=99.9% has 0.98us comprise app 1.95 times, which is much smaller than 7.8times 
 3. Any other reason LEDs are not turned off completely inspite of D=99.6%? 
 Thanks, 
 Kazu 
 PDF

Kazu · Accepted Answer

Thank you, 
 Kazu

Fil · Answer

Hi Kazu, 
 Apology for late reply. 
 1. D=99.6% and =99.9% are almost same so I expect the result to be almost same. It both cases, the PWM signal is telling the current through the LED to be almost ON the whole period so both LED should be ON. If you can get current waveform through the LED, you should be able to understand what's going on. 
 2. PWM input signal is related to turning LEDs off. It needs to be held low 100% to completely turn off the LEDs.

Electronic Design Services - PartnerZone

Question about an external PMW dimming for LT3922

Top Replies