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WEBENCH® Tools/PMP5112: Dimming

Tom Woscek
Prodigy 70 points

Part Number: PMP5112

Tool/software: WEBENCH® Design Tools

I built and using the PMP5112 SEPIC PFC LED Driver. With 75VAC input, the output is approx. 89 VDC with about 500ma of current. The string of LEDs get very bright and hot even with a thermal pad. I was told to change R22 from a 100K resistor to a lower value and add a variable resistor in series to dim the LEDS. It works but, I want to limit the maximum current to 300ma. Any ideas how to change the design? I'm thinking that R4 and R13 would be changed.

Thank you,

Tom  

 

  • 0
    TI__Mastermind 26695 points
    Hi Tom,

    Thanks for your interest in PMP5112. If I may suggest an alternative, install a 255kΩ resistor at R19 (assuming R22 is still 100kΩ). This should change the regulation set point of the TLV431 shunt regulator to be 4.4V and regulate the voltage across R2 to be 600mV.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 70 points

    Hi Ben,

    Thanks for the reply and the suggestion. I will try this. R22 was changed to a 50K fixed with a 100K variable in series to apply some dimming capability. I probably should go back to a fixed resistor of 100K if changing R19 will drop the current down to 300ma.  

    Best Regards,

    Tom

  • 0 in reply to Tom Woscek
    TI__Mastermind 26695 points
    Hi Tom,

    Did you get a chance to test with the new resistor values?

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 70 points

    Hi Ben,

    Not yet. Should be up to it in the next day or two. But, your idea gave me a thought for a new design change. I'm thinking I should go back to R22 as a fixed 100K resistor. What would you think if I installed a 50K variable resistor in series with R19 and use that to change the current from the 500ma to 300ma? The variable resistor is series with R22 has to be set fully CCW to zero first. If it is set close to 50K the circuit does not start. Putting in series with R19 may make it more stable.

    I'm finishing up another project and this is the next one. FYI, I built a LCD backlight tester for a customer 2 years ago and they now want another one. They are testing the board with 3 LED strings out of its case. When it is assembled there is a thermal pad and a heatsink that dissipates the heat. But, out of the case when these LEDs draw 500ma they are very bright and will get very hot. At 500ma it doesn't take long for them to melt the solder on the tabs. At 300ma the heat is less and they can be left on longer. This circuit is for 2 LED strings: one with 26 and one with 27 LXHL-PR09 LEDs. The 3rd string has 109 orange LEDs. I'm using TI ckt PMP6008 (TPS92560) for that string. Those LEDS draw a maximum of 70ma and can be left on with no worries of overheating out of the case.  

    Best Regards,

    Tom

  • 0 in reply to Tom Woscek
    TI__Mastermind 26695 points

    Hi Tom,

    Placing a variable resistor in series with R19 should be fine. I don't see any issues using this approach. Let me know how your testing goes when you have a chance.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Prodigy 70 points

    Good Evening Ben,

    Wanted to get this off to you before Monday morning. Yes, this is going to work much better than placing the variable resistor in series with R22. For my testing I left R19 at 221K (I didn't have a 255K), tombstoned it, and placed a 100K resistor is series to R18. Here's how it is working now; with the variable resistor full CW (0 ohms) the current through the LED string is 500ma just as designed. The voltage @ TP6 is 0.785v and @U2-2 3.98V. Turning the variable resistor CCW and monitoring the current it varies down to 150ma with the variable resistor @ approx. 70K ohms. At this point the voltage TP6 is 0.206v and @U2-2 4.73v. If I go any higher on the resistance the voltage @TP6 drops below 200mV and the LEDs flicker. So, I guess I should go with your suggestion on changing R19 to a higher value and place a 50K variable resistor in series instead of a 100K.

    Thanks much for this modification tip.

    Best Regards,

    Tom