I have 2 LED Driver designs built and am using: PMP5112 Rev E and TPS92560 _120V_Boost_A. They both work but want to know how to adjust the current to dim the LED strings. | |
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Hello,
Are these two separate designs?
It looks like current turned to voltage via R2 is fed back to Vsense. There is a pull-up of 100K, R20 and a series resistor with the voltage representing current of 100K, R22. Changing these values will change the output current. Vsense does need to be above the enable threshold of 0.72 volts (worst case) to start. That's one reason R20 is on the board. So sourcing current to the Vsense node will cause it to dim.
As for the TPS92560 board input voltage 120 VAC? Is it based on a TI design?
Thanks,
Thanks, Irwin
Yes, the LED string for the TPS92560 design is 109 orange leds in series of course and the max current is 70ma. It takes about 135vac input to get the 190v output before they light up. They operate from 190 - 230v. This ckt works great. So, yes I may try something such as a 250ohm variable pot with a 120ohm fixed resistor is series with it to vary the current. Good to know that going over 374ohms will increase the current. With this design I get a max of about 40ma and the LEDs are rated at a max of 70ma. So, I got a little bit of overage current to play with. Varying the input voltage will vary the current and brightness but, I don't want the techs having to vary the input voltage. I'm afraid they may kick it up too high. I want to set it at 135vac and then have them just turn some switches and turn a knob to see of the LED string changes brightness. These ckts are just for testing to make sure they operate before they go back to the customer and they install the assembly back in their final product.
About the other, PMP5112, yes that was exactly what I was thinking: replace R22 with a 100K pot to vary the current and brightness. Those LED strings are white LEDs and have a max of 1.4A. These are mounted on a heatsink with a thermal pad between them. At 250-300ma they get very hot when operated off the heatsink.
Best Regards,
Tom