IRS2540 Datasheet, PDF (19/28 Pages) International Rectifier – IRPLLED1 High Voltage LED Driver using the IRS2540

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Enable Duty Cycle Relationship to Light Output
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Fig. 23: Light Output vs. Enable Pin Duty Cycle
Fig. 24: IRS2540 Dimming Signals
Since the IRS254(0,1) does not include an onboard oscillator, a soft start feature is not possible. This
is only a concern when operating in the dimming mode. Since PWM dimming is required of LEDs,
the output is essentially turning on and off at a rate of the dimming frequency. In the absence of soft
start, a large spike of current would be observed in the load each time the output is turned on. This
current spike stresses the load possibly decreasing its overall lifetime. The IRPLLED1 includes a
jumper setting to define whether or not the board is being used in the dimming mode. This two
position jumper will allow the designer to either include or exclude the resistor R_out, which is in series
with the output capacitor. The inclusion of this resistor will sufficiently damp the output stage, such
that output current spikes are significantly reduced or eliminated. The presence of such current spikes
may cause the inductor to hum or buzz, the emitted sound will be that of the dimming frequency. The
inrush of current causes mechanical movement in the inductor which can be heard if the PWM signal
is within the audible range of the human ear. The effects of adding in R_out can be seen in Figs. 25 – 28.
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RD-0608
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