[Ken Shirriff] had to get down into a bit of semiconductor physics to give us an explanation of the TL431, which he calls “the most common chip you’ve never heard of”. [Ken] may well be right about the TL431. Even Texas Instruments can’t nail down a single name for it. Their page for the part calls it a “Adjustable Precision Shunt Regulator”, yet the datasheet is titled “Precision Programmable Reference”. You’d think they’d have figured this out by now, considering the TL431 was launched in 1978.
TL431’s can most often be found hiding in switching power supplies. The Apple II switcher had one, and many current ATX supplies have 3. Uninformed parts scroungers may miss them, as they often hide in TO-92 or SOT-23 packages. The TL431 is no transistor though. The TL431’s operation is actually pretty simple. When the voltage at the reference pin is above 2.5V, the output transistor conducts. When the reference voltage falls below 2.5V, the device stops conducting. In a power supply, this operation would help the control electronics maintain a stable output voltage.
The real subject of [Ken’s] article is the layout of the TL431 on its silicon die. Rather than bust out the fuming nitric acid himself, [Ken] uses some of [Zeptobars’] decapped chip images. Inside the TL431, [Ken] discovers that transistors aren’t made up of the three layer NPN or PNP sandwich we’ve come to know and love. In fact, the base isn’t even in the middle. Transistors, including the BJT’s used in the TL431, can be assembled in a nearly infinite number of ways.
[Ken] moves on to the resistors and capacitors of the TL431. The capacitors are formed two different ways, one as a reverse biased diode, and the other as a more traditional plate style capacitor. The resistors include fuses which can be blown to slightly increase the resistance values.
The takeaway from all this is that once you get down to the silicon level, it’s a whole new ball game. Chip layout may look a bit like PCB layout, but the rules are completely different. [Ken] mentions that in a future blog he’ll go into further detail on the operation of the TL431’s bandgap voltage reference. We’ll be watching for that one, [Ken]!