In response to Hans' message of a few days ago:
> LIN_REG_EN is connected to the supply voltage via a 100K resistor. When the on/off switch is first pressed and the CPU didn't yet boot up, the voltage will be somewhere between supply voltage and 3.3V. After the processor boots up and controls the pin, it will be 3.3V. After the SMPS boots up, it will be 0V.?
In my case, IC101 does not get any voltage from the drain of Q102 so never produces 3.3V to supply the STM32F446. The question is why does it not get turned on. I believe it is because Q101 is not turning on to allow current from the gate of Q102 to flow to GND thus meaning that Q102 does not supply voltage to Vin of IC101. I measure LIN_REG_EN at 860mV, this is very close to the minimum threshold gate-source value of Vgs(th) of the BSS123, which is 800mV, with a maximum of 2V and a typical value of 1.7V.
Hans says that LIN_REG_EN will be somewhere between the supply voltage (7V in my test case) and 3.3V.?When I removed the IC101 PSU board from the QMX and supplied it with 7V IC101 produced a solid 4V output and LIN_REG_EN was just a little below supply voltage. My conclusion is that something on the main board is causing the problem and I'm assuming it's the short from Vdd to GND, that is presumably inside the STM32F446.
However, 4V produced by the IC101 regulator is above the maximum specification for the STM32F446 operating voltage supply which is specified at 1.8-3.6V (page 92 of STM document RM0390). My subsequent conclusion is that the STM32F446 was fried causing the Vdd-GND short as soon as I supplied power to the board because the IC101 is supplying a voltage outside of spec. I'm happy to hear alternative suggestions but in reality all I really want is a working QMX.
Simon.
N.B. all voltage measurements were made with a Siglent oscilloscope.
