Using constant current to drive LEDs is a good practice, but turning them on instantly can be dangerous for both the LED and the LED driver. Harmful spikes are generated when large currents are instantaneously turned on. This app note from Maxim describes how to soft start LEDs to increase overall lifetime and performance.
An incandescent bulb requires some time to reach full brightness after you switch it on, and that delay gives the eye a comfortable interval for adjusting to the bright light. LED-based lights lack this property. Instead, their brightness goes from zero to 100% almost instantly. That property is welcome in a camera flash, but rather annoying for general lighting.
Maxim
APPLICATION NOTE 4481
APPLICATION NOTE 4481
By: Fons Janssen, Senior Member of the Technical Staff
Abstract:
To emulate the natural soft start of an incandescent bulb, this circuit makes use of the thermal-foldback protection built into an LED-driver IC (MAX16832). It thereby avoids the annoying and almost instantaneous startup otherwise exhibited by LEDs.View PDF
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