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LED driver ground: separate from digital ground?

Anonymous
Anonymous

Hi,

Does LED driver's ground needs to be separated from the digital ground?

In many designs I have seen, analog grounds are separated for DACs, ADCs and various components with an inductor connecting them. The reason for this is that generally digital domain has more noise which if not isolated, would have negative impact on analog signal qualities.

If the application require uniform LED light, then is it necessary to separate LED driver's ground from the common digital ground?

But it seems that there is a problem with ground isolation: PWM. Many TI LED drivers accept PWM  as brightness/dimming control signal. For PWM to work, it seems more natural and typical for LED driver to share the same GND signal as the CPU, though it is also quite likely that grounds separated by an inductor would also allow PWM signal to pass in.

The fundamental questions is that how sensitive is LED drivers to digital noise? In TI's LED driver selection tool, LED drivers are usually marked as having accuracy (current, for example) to as low as 0.5%. Is the current immune to digital noise? If there is heavy noise in the digital domain, how much would it affect LED driving current?

 

Garry

 

 

  • 0
    TI__Guru**** 166916 points

    It is difficult to fully charactertize the effect of digital noise on a circuit.  So, where possible we always recommend having the analog circuitry (e.g., ground of current/voltage setting resistors) separate from a digital ground plane with the two grounds connecting only through a small (and therefore high impedance) trace.

  • Anonymous
    0 Anonymous in reply to Jeff F

    Jeff,

    Another way, of course, is to use optocoupler or digital isolated, but I believe it is too much an overkill for just a LED driver.

    But for LED drivers, do you typically do that? If you separate the ground, how do you pass PWM signals to LED driver?

     

    Garry Hodges

     

  • 0 in reply to Anonymous
    TI__Guru 65450 points

    Hello Garry,

    As Jeff mentioned, the noisy GNDs like digital or power GND are normally connected to the analog GND with a higher impedance node. In general there is no need for using optocouplers or digital isolators between the CPU and the LED driver.

    We also saw good performance with solid GND planes and separated GND paths for the noisy signals. You just have to make sure that the potential on all GND paths stays the same, e.g. by using vias.

    Best regards,
    Brigitte