# Powering TL431 from MCU output possible?

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?

• It is possible. Consider that the power supply labeled 3.3V, might be 3V real. It's possible to use a transistor connecting to ground and it's possible to use the GPIO pin directly. You have to check what will be the state of the pin in sleep mode. – CFCBazar com May 13 at 18:41
• We are using a similar thing in our electronic electricity meters, to reduce the power consumption while supplying the control circuitry from the internal non-rechargeable battery which is the only power source for extreme situtations such as power outages. No problems so far. – Rohat Kılıç May 13 at 19:21

If the supply is 5% low (3.135V), the resistor 5% high (409.5$$\\Omega\$$), and the reference at the maximum 2.550 the current will be 0.99mA. So maybe use a 1% resistor, but 5% will be fine too.