To drive this mirror, the voltage output necessary is less than 2 V for the maximum amount of dimming. Due to the size of this capacitive load, the current necessary to charge it can reach as much as 400 mA. A steady state of approximately 150 mA is also required to maintain proper dimming. To meet these specifications, use an op amp and a Darlington bipolar junction transistor (BJT) to buffer from the output of the Sallen-Key filter and drive the electrochromic load. The BJT sources the current for the load and the use of an op amp feedback provides stability and linearity regardless of the temperature or process variation of the BJT.

The MJD122 Darlington-pair BJT is chosen for its large 100-V collector-emitter voltage to allow use across both the typical and non-typical automotive battery operating voltage range, as well as the very-large, 8-A continuous current operation. The DPAK package is great for dissipating heat generated by the typical 2 W of power flowing through the device.