[sdiy] CV input op-amp circuit

Mike Beauchamp list at mikebeauchamp.com
Mon Dec 7 21:15:55 CET 2020

On 12/5/20 11:38 PM, Brian Willoughby wrote:

> I'm not quite sure how your *external* protection diodes could cause any problems like this, but it's potentially similar to other problems.

You know, I'm not sure either..

I've attached the relevant part of the schematic.  The +3.3V-D is coming 
directly from the Teensy 3.2 onboard Voltage Regulator (fed from a +5V 
regulator - which is fed from a +15V regulated rail).

TUNE-RAW is expected to be -4.1V -> 0V
OSC2-TUNE is expected to be -10.2V -> 0V

My Teensy fails to start properly about 50% of the time like this. When 
I remove both BAT54-S's (or disconnect them from +3.3V-D) things start 
up normally.

 > In your "fails 50% of the time" are you able to wait for power to 
 >completely drain from the supply rails as well as all other parts of 
 >the circuit? Sometimes a resistive load on supply capacitors can help 
 >drain them when you want to recover from a user that flips the power 
 >switch too quickly.

Thank you for mentioning this problem as well Brian! I didn't know it 
was a thing.. but while testing my BAT54-S problem I also noticed that 
if I powered the unit on at a very specific timing after power-down, the 
microcontroller would fail to start 100% of the time. With the scope I 
could see it was always when the microcontroller supply voltage (which 
has a dedicated 10uF cap on it) would drain to a certain range.


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