delivered to the LED and RSENSE2 voltage becomes zero. During this dimming off-time, EA output increases to its maximum and overcharges the EA compensation network. When the modulated PWM turns on again, it takes several cycles before it recovers while high-peak current is driven to the LED. This current peaking scenario shortens the lifetime of the LED.
To avoid this problem, the LED dimming engine allows the PWM3 to be used as an override source of the OPA. When the PWM3 is low, the output of the EA is tristate which completely disconnects the compensation network from the feedback loop and holds the last point of the stable feedback as a charge stored in the compensation capacitor. When the PWM3 is high and the LED turns on again, the compensator network reconnects and the EA output voltage immediately jumps to its previously stable state (before PWM3 is low) and restores the LED current set value almost instantly.
As mentioned earlier, a LED dimming engine can operate with minimal to no CPU intervention. Therefore, since all work for controlling the LED driver is offloaded to the CIPs, the CPU has significant bandwidth to execute other important tasks.
In Figure 2 the microcontroller senses the input and output voltage. Out of these sensed voltages the LED driver employs protection features such as undervoltage lockout (UVLO), overvoltage lockout (OVLO) and output overvoltage protection (OOVP). Implementing this protection through software ensures that the LED driver is operating within desired specifications and the LED is protected from abnormal input and output conditions. The CPU can also process the thermal data from a sensor to implement a LED’s thermal management. Moreover, when setting the dimming level of the LED driver, the CPU can process triggers from a simple external switch or command from a serial communication. Also, the parameters of LED driver can be sent to external devices through the serial communication for monitoring or testing.
Aside from the features mentioned above, the designer has the luxury to add more intelligence on their own LED application inclusive of communications, like DALI or DMX, and control customizations.
An 8-bit microcontroller can be used to create an effective LED driver. Aside from providing efficient energy source, ensuring LED’s optimal performance, and maintaining the long life of LEDs, the microcontroller can add advanced features which make the lighting solution even more attractive.
Mark Pallones is a Principal Engineer and Team Manager of the Applications Engineering Group MCU8 division in the Philippines for Microchip Technology Inc. He has authored and co-authored several application notes and technical articles and has been published on the Microchip website as well as technical publications across the world. He earned his post graduate degree in Electrical Engineering in Power Electronics at the University of the Philippines and his undergraduate degree in Electronics and Communications Engineering at New Era University.
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