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Analog switch adds strobe to dimmable LED driver

Flashing LEDs are all around us, as warning lights on automobiles, as indicators on appliances, as two examples, and the information they convey is greatly enhanced when their light intensity is variable as well. Flashes of different intensity can indicate different levels of warning in an automobile, or how soon an appliance will complete its job. A continuous modulation of brightness can provide interesting lighting effects.

Adding a simple analog switch (U1, MAX4645) to a standard PWM/analog LED driver with dimming capability (U2, MAX1561) allows you to both flash and dim the LED (Figure 1 ).



Figure 1: An analog switch (U1) adds a strobe function to this standard PWM LED driver circuit

(Click on image to enlarge)

The LED driver's CTRL input has a dual purpose in this circuit. As a PWM/analog input it controls current through the LEDs, and as a shutdown input it deactivates the driver when brought low for more than 8.2 ms. A pulldown resistor at CTRL discharges the pin capacitance to ensure that VCTRL is brought low enough to trigger shutdowns.

By allowing you to alternate between these CTRL functions (dimming control and shutdown), the analog switch enables a flashing, dimmable LED driver. Figure 2 and Figure 3 show operation of the circuit using the voltage at CS as an indicator of LED current, for the conditions of 40% PWM strobe and 80% PWM strobe, respectively. ♦



Figure 2: Operation of the Figure 1 circuit at 40% duty cycle with a 5 Hz strobe, as depicted by the voltages at U2's CTRL and CS inputs.

(Click on image to enlarge)



Figure 3: Operation of the Figure 1 circuit at 80% duty cycle with a 5 Hz strobe, as depicted by the voltages at U2's CTRL and CS inputs.

(Click on image to enlarge)

2 comments on “Analog switch adds strobe to dimmable LED driver

  1. Amazulu
    July 9, 2013

    I am trying to achieve the same result using the Meanwell LDD-1000H module and a MAX323 switch. Giving me some weird results so I'm wondering if the LDD is having a timing issue on its DIM input? Not much techn info on this part.

  2. Amazulu
    July 9, 2013

    any idea?

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