The way to achieve dimming function can be divided into two types: Analog and PWM.
Analog dimming technology, just the white LED current is reduced to half of the most too value can make the screen brightness is reduced by 50%. The disadvantage of this method is the color shift of the white LED requires analog control signal.
PWM mode dimming techniques provide a complete in reduced current duty cycle current to the white LED, for example, To reduce the brightness half, provided within the full current just in a 50% duty cycle. The frequency of the PWM signal will usually exceed 100Hz, in order to ensure that the pulse current is not the human eye to perceive to the PWM frequency of the most too value in accordance with the startup of the power supply and the reaction time is determined. While allowing for maximum flexibility eve achieve easier, white LED driver can accept up to 50kHz PWM frequency. Dimming signal is usually from the system microprocessor GPIO pins.
White LED applications, if there is an open circuit failure, constant current white LED driver needs overvoltage protection. The white LED and the drive is typically located in a different circuit board, therefore the connector pin loose will cause the open-circuit fault, and another possibility is a white LED failure to open. Matter what kind of situation, the drive in order to provide constant current, will increase its output voltage. The absence of an overvoltage protection circuit this case, the output voltage will rise quickly, and cause damage to the drive or output capacitance.
The easiest way to protect the drive is to select the built-in white LED drive voltage comparator, and take advantage of this feature to limit the maximum output voltage, for example, the TPS61043 you have over-voltage protection function. Zener diode can also be used to limit the maximum output voltage, the efficiency of this method, however, is very low, because there will be a pre-set maximum current under fault conditions through the Zener diode.
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