On Tue, Aug 29, 2023 at 11:54 AM, Chris wrote:

Nothing.? The Linear continues to produce 3.3V relative to it's 'GND' pin. if the supply produced by the SMPS is more then it provides nothing.? All this is confused by D102 andD103 which prevent the linear getting a voltage on its output which it can't cope with.

I did a poor job of stating my question. ?I agree with your assessment of the voltage regulator output. ?My question was about the LIN_REG_EN signal. ?If you look at the trace below, channel 2 is LIN_REG_EN as measured on the 3v3 board connector, i.e. downstream of R101 and Q101. ?It is initially around 7.5V, then drops to 3.3V, then to 0 when the SMPS comes online. ?I'm trying to understand the mechanism that pulls LIN_REG_EN FROM ~7.5V to 3.3V. ?Or for that matter why it is initially at 7.5V instead of the supply voltage of 9V.



Secondly, since I was measuring the voltage right at the connector, it seems that the processor pin would also have been exposed to that 7.5V. ?Would that be a problem? ?Obviously the current would have been negligible due to R101.

Jonathan KN6LFB