I'm designing a board with ATmega128 MCU. I have a TL431 reference connected to the ADC_REF pin, with both TL431 and the MCU connected to the same supply, 3.3 V.
Now I realized one problem - since the TL431 is connected directly to the 3.3 V, I don't have any possibility to switch it off to reduce power consumption, while the MCU is in sleep mode.
My solution, I came up with so far, is to power the TL431 from the output pin of ATmega, via a 390R resistor. This would allow me to switch off the reference before the MCU goes into sleep mode. At 3.3 V at the output, the cathode current would be 2 mA, which is sufficient, it should even work for output voltage as low as 2.9V (1 mA cathode current).
Are there some possible problems with this solution?