TL866 programmer schematic question
See the image below, and notice that L4's VCC will not be lower than ( 5V - V_D6 ). With VDD less than 5V, L4, L3, and L2 will receive power from the 5V rail instead. So with L4's VCC at approximately 4.3V, you don't violate the part's limit on V_IN - V_CC