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In SSD1306 the segment current sets with resistor at IREF pin, and the current depends on VCC.

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I have no stabile VCC, because it powered either directly from 9V alkaline battery or external 12V source, and I don't want create my own DC/DC or use internal charge pump (it too big consumption according the OLED's datasheet).

But I already has 3.3V stabile power source. So, can I use the following solution to create stabile IREF ~ 12.5μA (and therefore stabile brightness)?

enter image description here

And second question. Can I get any other trouble because of using not stabile VCC (but in the range from the SSD1306's datasheet)? VDD for logic is stabile 3.3V.

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Yes, that should work provided the Vcc > 6V or so so the current sink does not saturate.

You might want to put a (series) base resistor on the transistor in case something strange is going on inside the chip, something like 10K will do. For example, if your 3.3V supply ended up (under some transient condition) being higher than the Vcc, then current could flow into the SSD1306, which might be destructive.

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  • \$\begingroup\$ Thanks! About base resistor: I get 3.3V using LDO regulator from VCC, so base voltage is always lesser than VCC. The budget is regulator's dropout (about 0.3V) + 0.6V: I think it's sufficient for transients. \$\endgroup\$
    – user54579
    Jul 17 '14 at 15:23
  • \$\begingroup\$ I'm not convinced you're 100% safe- if there is a heavy load on Vcc when power is cut, it could drop much faster than the 3.3V bus. \$\endgroup\$ Jul 17 '14 at 15:53

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