A die photo of the interesting but little-known TL431 power supply IC provides an opportunity to explore how analog circuits are implemented in silicon. While the circuit below may look like a maze, the chip is actually relatively simple and can be reverse-engineered with a bit of examination. This article explains how transistors, resistors, and other components are implemented in silicon to form the chip.
The TL431 is a “programmable precision reference” and is commonly used in switching power supplies, where it provides feedback indicating if the output voltage is too high or too low. By using a special circuit called a bandgap, the TL431 provides a stable voltage reference across a wide temperature range. The block diagram of the TL431 below shows that it has a 2.5 volt reference and a comparator, but looking at the die shows that internally it is quite different from the block diagram.
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