I have an changing input voltage. I would like to feed it into LM3406 and have varying brightness from the Led. So I go through the datasheet.

However,the only information and characteristic of the IC I can understand is that the duty cycle of the output is Vout/Vin when PWM dimming is being applied. I dont know how to obtain desired Vout.

I am not familiar with LEDs. The datasheet tell me high power LEDs can be used. What is high power Led actually. I can find many types of LED on the street and I dont know the compatiblity of each one.

In my case , is it dim pin is not necessary to connect anything? I find that that pin is left open in input voltage comparator for PWM dimming.


1 Answer 1


The DIM pin is used for PWM dimming from a control signal:

Connect a logic-level PWM signal to this pin to enable/disable the power MOSFET and reduce the average light output of the LED array. Logic high = output on, logic low - output off.

So if you connect a 50% PWM signal to the dim pin, the led connected to the driver is dimmed to 50%.

The VINS pin is used to detect PMW dimming on the Voltage input:

Adding an external input diode and using the internal VINS comparator allows the LM3406 family to sense and respond to dimming that is done by PWM of the input voltage. This method is also referred to as "Two-Wire Dimming", and a typical application circuit is shown in . If the VINS pin voltage falls 70% below the VIN pin voltage, the LM3406 family disables the internal power FET and shuts off the current to the LED array. The support circuitry (driver, bandgap, VCC) remains active in order to minimize the time needed to the turn the LED back on when the VINS pin voltage rises and exceeds 70% of VIN. This minimizes the response time needed to turn the LED array back on.

It uses a diode and a capacitor to smooth the input, and allow it to dim that way. See 8.11 for details.

If you want to take a steady but variable voltage input, and dim the led by the range of the input, you would need a circuit to measure the voltage, and produce a PWM signal to the DIM pin. Any common microcontroller should do.

  • \$\begingroup\$ Sorry, I think I am still confused. Can I rephrase it 8.11 as the increasing input voltage Vin can generate an output PWM signal increasing in duty cycle connecting DIM pin with a constant PWM signal? \$\endgroup\$
    – IssacLeung
    Commented Jul 16, 2015 at 16:32

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