The implementation of the dimming function can be divided into two types: analog mode and PWM mode.
When using the analog dimming technology, you can reduce the brightness of the white LED by 50% by reducing the current of the white LED to half of the maximum value. The disadvantage of this method is that the color shift of the white LED requires an analog control signal.
The PWM mode dimming technique provides a complete current to the white LED during a reduced current duty cycle, for example to reduce the brightness by half, providing a complete current in a 50% duty cycle. The frequency of the PWM signal usually exceeds 100 Hz to ensure that this pulse current is not perceived by the human eye. The maximum value of the PWM frequency is determined by the start-up and reaction time of the power supply. For maximum flexibility and ease of implementation, white LED drivers can accept PWM frequencies up to 50kHz. The dimming signal is typically from the GPIO pin of the system microprocessor.
In white LED applications, a constant current white LED driver requires overvoltage protection in the event of an open circuit fault. White LEDs and drivers are usually located on different boards, so loose connectors on the connectors can cause open-circuit faults, and another possibility is an open LED. In either case, the driver increases its output voltage in order to provide a constant current. At this time, if there is no overvoltage protection circuit, the output voltage will rise quickly, causing damage to the driver or output capacitor.
The easiest way to protect the driver is to select a white LED driver with an overvoltage comparator and use this feature to limit the maximum output voltage. For example, the TPS61043 has overvoltage protection. Zener diodes can also be used to limit the maximum output voltage. However, this method is inefficient because there is a preset maximum current through the Zener diode in the event of a fault.
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