Hi All,
I have a couple designs that use the TPS92641 to drive a single high-current (15-20A) LED under the control of a microcontroller. One of the designs uses a single TPS, the other uses 3. The single channel design works well. I perform a mix of manipulating the a filtered PWM voltage applied to IADJ and PWM chopping of SDIM to control the LEDs' output over its full range from around 1% to its full output at 100% (up to 20A). For higher output levels, I leave SDIM high and vary the voltage applied to the IADJ input by controlling the PWM duty-cycle. As LED desired light output drops, I hold IADJ at about 10% its nominal value and chop SDIM to further reduce light output. I am working with a couple of different wavelength LEDs, which both show this issue.
On the three channel board, I am having some intermittent issues with two of the channels (one channel works fine) but the other two intermittently shut down in spite of the fact that their control inputs are being properly driven. I am sure this isn't an overheating problem since the controller chips don't get warm to the touch. When not operating the controllers holds all its gate output signals in their inactive state. The 8.5V supply is working and the VREF is 3V as it should be. The UDIM input is biased to the controller's supply with a pair of resistors that set the brownout threshold to about 10V. I am running the boards from a 12V @ 20A or 12V@25A supply which always measures 12V.
When I increase the IADJ input to nearly 100% (1.78V), the chips begin working. If instead I increase IADJ to 20% or 30%, the chips remain dormant, as if their internal clocks have stopped. Once the IADJ input has gone high enough to get the chip "started", I can reduce it to 10% or 20% and the controller remains on. It will however, intermittently, after minutes or longer shut off. If I immediately increase IADJ to 100%, the controllers resume operation as before.
Two of the controllers on the three channel board do this. I have been over the boards dozens of times with a microscope and oscilloscope looking for the intermittent solder connection or missing signal and don't find one.
Appreciate any thoughts you might have.
Thanks,
Scott