I'm overlooking something really simple, and figured some fresh eyes will make short work of this issue!
I've just assembled the first prototype of my device, which is a 4-layer board, populated one both sides, with a total of ~800 components. As this is the first attempt at this design, I've come across a few small errors and areas for optimisation (as I would expect). One such area is in how the power management of the device is enabled. I'm using a TPS51120 PMIC, which manages two power rails (3V3 and 5V). The TPS51120 has a number of enable pins, and without going into detail, I have EN2 and EN5 connected together, which need to be driven high (they're both 5V tolerant). EN2 enables the 3V3 rail (EN5 has a role as well, but it's not as critical as EN2), whilst the 5V rail is enabled via EN1, which is toggled by a pin on my MCU.
As part of this original design, I incorporated a pushbutton control IC, the output of which was connected to EN2 and EN5. The pushbutton IC is connected to a switched power source, such that when the switched source is turned on, the pusbutton IC would send the required enable pins high to turn on the 3V3 rail. When the switched source was turned off, the pushbutton IC would send an interrupt to the MCU to allow proper powering down, before turning off the device. I've now removed these components from the prototype board, and would like to manually turn on/off the 3V3 rail using a simple rocker switch connected to the 12V source. As such, I've got the following:
The values of the resistors chosen are based on both bringing the 12V source voltage down to less than 5V, and what I had available. The voltage divider output is ~4.4V, which is a suitable HIGH logic level for both EN2 and EN5. Here's where I become thoroughly confused. With the 12V source applied, I have verified that the voltage divider output is indeed 4.4V. However, when I connected a wire from the voltage divider output node to the EN5 pin (which in turn is connected to EN2 via a trace), the voltage drops to ~0.78V. I even replaced R2 with a second 680Ohm resistor, which correctly produced a ~6.04V output when the EN2/EN5 wire was disconnected, but again dropped to ~0.8V when connected.
What am I missing here?