![]() Our duty cycle would need to drop to 14.167% (1.7V divided by 12V) and our minimum PWM frequency would decrease to 14.285kHz (the inverse of ). However, if our voltage was 12V, we would have to rework our calculations to keep the same amount of current flowing through the LED. If we use the LED I was referring to earlier, we know that 34% duty cycle, at 33.3kHz, at 5V is safe. I'll demonstrate my point by example instead of via explanation. Some components will be more forgiving than others, and, if you are lucky, the component's datasheet will specify how well it can handle spikes of current. NOTE: All components can safely handle temporary current spikes above their maximum ratings as long as the duration of the current spikes are SMALL ENOUGH. Then, I can be 99.99999% certain that I can safely operate the LED at 100mA for some period longer than 10us, probably close to 20us. For instance, with my LED, if I can operate the LED at 40mA for ever (40mA is the steady current max rating) and I can operate the LED at 200mA for 10us. They don't spec those scenarios because the vendor doesn't want to invest time in speccing and supporting the use of their component for corner use cases. The reason is this: Datasheets usually don't specify all of the valid operating scenarios of a component. ![]() In REALITY, I could safely use PWM to power my LED with a slower PWM frequency. I would need a PWM frequency 33.3kHz or greater to safely drive my LED (33.3kHz = The inverse of ). 58mA exceed my LEDs steady current maximum of 40mA by 18 mA. During the on-time, the current through the LED will likely rise to around 58mA (58mA = typical current draw at 1.7V of my diode divided by 34%). With a duty cycle of 34% and a power supply of 5V (34% of 5V = 1.7V) will produce an average voltage of 1.7V, I just need to ensure my PWM on time is 10us or less. I can drive the LED with 1.7V (the LEDs typical forward voltage from the datasheet). However, it also has a "Peak Forward Current" rating of 200mA, with a note that the current cannot remain at 200mA for longer than 10us. For example, the LED I have next to me has a max current rating of 40mA. BUT!!! If that information is specced on the datasheet, then you can safely take advantage of it. If that info is not specified in the datasheet, then the datasheet authors were lazy. Your biggest concerns will be how the LED handles a temporary high current and how the output circuit of your chip can handle a temporarily high current. When using PWM in this scenario, you are relying on the ability of your circuit's components to temporarily handle high currents.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |