App note: Soft-Start in LED lighting

Posted on Saturday, February 25th, 2012 in app notes, LEDs by Arup

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.

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4 Responses to “App note: Soft-Start in LED lighting”

  1. rsdio says:

    An LED cannot be harmed by instant-on power; this article says nothing of the sort; PWM circuits turn an LED on and off hundreds or thousands of times per second. The entire gist of this article is that humans find instant-on lighting annoying, and we generally only put up with it for camera flashes. I did not see any mention of dangerous current spikes.

    • Arup says:

      LEDs can’t get harmed by instant-on power as we normally see in practical world.
      But an LED is simply a p-n junction diode which produces light. Applying voltages greater than the forward voltage of the p-n junction we light the LEDs.
      Theoretically there is high I/V value at that range of voltage where we light the LEDs.
      The appnote doesn’t say that, but going from 0 to the high current instantly creates a spike in the conductor being used. You can find an inductor (in series) in most LED drivers whose main purpose is to absorb the spike. But with a moderately soft-start in LEDs, the inductor can be omitted as in that approach there will not be that huge spike.

      • Sleepwalker says:

        Indutors don’t produce a voltage spike at turn-on, there is no energy in the inductor at that time to produce a spike. The spike occurs at turn-off, where the inductor will oppose the fast rate of change in current.

        Also, in this circuit, the inductor is used as part of the step-down switching circuit, it’s not there for protection.

      • rsdio says:

        After 200 ms, this circuit still applies instant-on current to the LED. Thus, while you might argue that the soft-start protects the LED for a little while during the first 200 ms, it does nothing once the LED reaches full brightness when it is switching from full on to full off many times per second.

        The MAX16832 chip in this circuit has a 0.45 Ω 65 V DMOS FET that repeatedly grounds the inductor and then turns off again, thus exposing the series LEDs in the circuit to instant-on current several times per second. Once the brightness reaches full at 200 ms, this circuit continues to slam the LED with exactly the kind of current that you claim will harm the LED.

        Nearly all LED circuits employ PWM, which means that nearly all LEDs are being subjected to square wave control signals that switch between full off and full on current several times per second. I do not understand how you can protect an LED from instant-on current or power when switching under square wave control. I believe you are mistaken.

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