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Sinlge LED drive consant current drive

Calum Mackinnon
Genius 4155 points
Other Parts Discussed in Thread: TLC5916, LP5524, LP5522, LP5560

Hi,

I need to drive a white OSRAM LED that is remote from the current source(ie there is no easy way to get the feedback and measure the current at the LED. I need  a device that will give me a constant curremt of about 5mA however I do need a way to adjust the brightness a little. Analog or digital control for brightness would work.

I will probably have a 5v input rail/

Any help appreciated.

Calum

  • 0
    TI__Genius 14890 points

    Calum,

    You might try the TLC5916.  It is an 8-channel (7 more than you need) constant current LED Driver.  It has a register available for dimming.  This is a serial interface device (SPI).  The TLC5916 can run from 3V to 5.5V without issue.

    Regards,

    Dick

  • 0
    TI__Intellectual 2510 points

    Calum,

     

    What kind of package do you prefer? If an BGA is suitable for your application you could check out LP5524, which has 4 LED outputs 5mA default current and 2 channels for external PWM control. This in uSMD package.

    There is also 1 LED output drivers LP5522 and LP5560 which can be trained to give different kind of blinking sequences. These are also in uSMD package.

    Regards,

    Sami

  • 0
    Prodigy 60 points

    Hi Calum, 5mA is not going to give you much in the way of brightness from any LED. You would need at least 20mA to get anything reasonable.

    You do not say what the power rating of your LED is but a simple 1/2 Watt LED can give you a good light output at even 50mA. 5mm standard LEDs should not be run higher than 25mA.

    Using a simple linear regulator with an adjustable output should allow you to generate a contant current irrespective on the length of the wire as any voltage drop in the wire is compensated for at the source (the regulator). It is only interested in what current is flowing in the loop.

    The feedback voltage is what controls it all.

    If you have a feedback voltage of 1.25V and you want a current of 25mA, then ohm's law says you need a value of 1.25v / 0.025A = 50 Ohms as your sense resistor, which is connected in series with your LED to ground. The junction of the LED and the resistor is connected to the feedback input of your regulator and voila - constant current. The equations are there, just modify for your regulator feedback value and LED current requirement as required.

    There are many simple 3 terminal linear regulators available that can do what you require, just look for the ADJ types rather than the fixed output versions.

    You can go as far as a switch mode regulator which would allow higher currents but the complexity of this solution is not what I think you need.

    Good luck with your project.

    Regards, Dave Neale