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TPS92641: Driving LED arrays with long cables using SDIM PWM

Wesley Lee Yang
Prodigy 20 points
Part Number: TPS92641

Hi,

I'm designing some boards using TPS92641 to drive LED arrays (~40Vf and ~2A). There is one installation in particular where it would be considerably easier if the boards are installed far away from the LED arrays (circa 15m~20m away).

In this application, SDIM is preferred due to the nice control during low duty cycle PWM. (Not only in one LED array but also amongst LED arrays when they slowly turn on)

Besides the resistive losses and voltage drop on the cable, are there other issues that I have to take into consideration? My main worry is that the extra parasitic inductance in the cable would pose a problem or instability. I'm not entirely sure if EMI is an issue right now because this is a one-off project that will not be commercialized. I understand that the shunt FET wont be able to shunt the current on the parasitic inductance of the cable but I think this side effect will be tolerable.

I can adjust the Switching Frequency, PWM frequency and the inductor value if there is a range that is more suitable for this (and other modifications to the circuit as well).

Thanks in advance

  • 0
    TI__Mastermind 28655 points
    Hello,

    Shunt Fet dimming a load that far away can cause issues. It requires understanding how much inductance is in series with the load and how it behaves when Shunted and Opened. This may requires clamps or dampers to prevent over voltage conditions. High frequency Shunt FET dimming probably won't work at that distance. Controlling the on/off slew may allow it to work but it will add switching loss to the Shunt MOSFET. The resistive losses, unless they become rather large, should not be an issue. It will just add resistance to the dynamic impedance of the LED which should be fine.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 20 points
    Hmm, the inductance will be varied since the cable lengths will vary. I guess it would be better to stick to "regular" dimming through UDIM then. Would it deal better with the extra inductance? And what would I have to be careful about?

    Thanks for the help
  • 0 in reply to Wesley Lee Yang
    TI__Mastermind 28655 points
    Hello Wesley,

    Regular dimming will behave better. If you are driving an LED load with inductance between the driver and load you will want to watch for voltage excursions exceeding component ratings or causing currents to flow where you don't want them to flow. If you decide on using output capacitors it is important where they are located and how the 'LC' behaves when turning the driver on and off. You may want to simulate what will happen when turning the current on and off (both voltage and current) if there are output capacitors. Without output capacitors the cable inductance is in series with the LED driver inductor. Depending on what the output voltage looks like at the output of the LED driver it could affect the Vout measurement by the IC.

    Best Regards,