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.
The TL431 has a long history; it was introduced in 1978 and has been a key part of many devices since then. It helped regulate the Apple II power supply, and is now used in most ATX power supplies as well as the the iPhone charger and other chargers. The MagSafe adapter and other laptop adapters use it, as well as minicomputers, LEDdrivers, audiopower supplies, video games and televisions.
Read the thoroughly researched article on Ken’s blog.
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