Re: QMX - smoke - another C107/Q108 failure
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The 1N4148 diode in series with the linear 3.3V output is a necessary evil to prevent backward current flow once the 3.3V SMPS rail comes up. However, it destabilizes the linear 3.3V rail as noted by the variation in voltage as the processor is executing its boot code. It is advisable to have a substantial capacitance on the processor’s power pins to minimize voltage. This should be at least 47uF - 100uF to ensure that the processor doesn’t experience an instantaneous dip below its minimum spec voltage which could cause the boot process to hang or go off into never-never land.
What happens on one unit is not representative of 1000’s of units so I hope the processor voltage during bootup is being tested on every unit prior to shipment.
Hi Jonathan, Kees
These plots supplied by Jonathan look completely normal to me.
The 3.3V rail experiences some disturbances during the boot-up process and this is completely normal; it is powered by the 78M33 linear regulator during that time via a 1N4148 diode. The diode voltage drop will naturally?show some variation depending on current draw, and current draw will be variable as the unit boots up various things.? On Mon, Aug 28, 2023 at 6:27?PM Jonathan Burchmore < burch@...> wrote: Kees,
Here you go.? Measured on my working 9v QMX.
On:
Turn off with encoder press:
Hard turn off (in this case turning off the bench power supply, not actually pulling the plug):
Hope this helps,
Jonathan KN6LFB
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