Constant Current LED Bank Controller

The top left PNP is serving as the current mirror that controls the rest of the PNPs to the right. The bottom NPN is typically connected to a 3.3V uController Push Pull output pin (or a PWM capable output to fine-tune the current). This bottom NPN allows for a constant current calculated (roughly) as: Typical drop of a NPN from Vbe is 0.6V. According to this simulation, it seems the Vbe of these simulated NPNs are closer to 0.85V. The bottom resistor is typically adjusted to match the desired current output. (3.3V - 0.6V) / 60 Ohms = 45mA constant current, which is then current mirrored using the PNP configuration at the top.

published 5 years ago