How can I current-limit a current mode DAC?
Your MCU output is limited to 3.3V maximum, but the OLED display's IREF input sits at ~7V. Therefore as well as reducing the current you will also need to extend the voltage acceptance to >7V. Here's one solution:-







simulate this circuit &ndash; Schematic created using CircuitLab

The MCU's IDAC is configured to source up to 64uA, which is converted to a voltage by R1. OA1, Q1 and R2 create a constant current sink. The op amp maintains the same voltage across resistor R2 as R1, but since R2 is 5 times larger the current through it is 5 times smaller (ie. 12.5uA vs 64uA). Q1 is rated for 45V so 7V is no problem for it.