Soft start is used to ensure the switching regulator does not over shoot the current into the LED at initial turn on. There is an internally set 500 µs delay due to the internal low pass filter. The start delay time can be increased by adding a capacitor to the ADJ in to ground. The delay will increase by the rate of 0.5 ms per nF. You cannot easily use the soft start as a slow start up brightness control for the following reasons. Firstly, the shut down pin enables at around 200 mV so the on current will be about 20% of the set current so it turns on with significant brightness. Also due to logarithmic nature of the eye linear changes in current are not easily noticed. If soft start brightness control is required low frequency Pulse Width Modulation is the best method. This will be discussed later.