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Get To Know Voltage Regulators with a 723

Get To Know Voltage Regulators with a 723


“Chapter 5; Horowitz and Hill”. University students of all subjects will each have their standard texts of which everyone will own a copy. It will be so familiar to them as to be referred to by its author as a shorthand, and depending on the subject and the tome in question it will be either universally loathed or held onto and treasured as a lifetime work of reference.

For electronic engineers the work that most exemplifies this is [Paul Horowitz] and [Winfield Hill]’s The Art Of Electronics. It definitely falls into the latter category of course books, being both a mine of information and presented in an extremely accessible style. It’s now available in its third edition, but the copy in front of me is a first edition printed some time in the mid 1980s.

The Art of Electronics, on regulators.

Chapter 5 probably made most of an impression on the late-teenage me, because it explains voltage regulation and power supplies both linear and switching. Though there is nothing spectacularly challenging about a power supply from the perspective of experience, having them explained as a nineteen-year-old by a book that made sense because it told you all the stuff you needed to know rather than just what a school exam syllabus demanded you should know was a revelation.

On the first page of my Art of Electronics chapter 5, they dive straight in to the μA723 linear voltage regulator. This is pretty old; a design from the legendary [Bob Widlar], master of analogue integrated circuits, which first made it to market in 1967. [Horowitz] and [Hill] say “Although you might not choose it for a new design nowadays, it is worth looking at in some detail, since more recent regulators work on the same principles“. It was 13 years old when they wrote that sentence and now it is nearly 50 years old, yet judging by the fact that Texas Instruments still lists it as an active product without any of those ominous warnings about end-of-life it seems plenty of designers have not heeded those words.

So why is a 50-year-old regulator chip still an active product? There is a huge range of better regulators, probably cheaper and more efficient regulators that make its 14-pin DIP seem very dated indeed. The answer is that it’s an incredibly useful part because it does not present you with a regulator as such, instead it’s a kit of all the parts required to make a regulator of almost any description. Thus it is both an astonishingly versatile device for a designer and the ideal platform for anyone wanting to learn about or experiment with a regulator.

Block diagram of a 723 regulator. From the TI data sheet.
Block diagram of a 723 regulator. From the TI data sheet.

Running through the package contents, there is a temperature compensated voltage reference, an error amplifier, an output transistor, and a current sense transistor, all presented almost as separate components on a blank slate for the designer. It can be configured as a negative or a positive voltage series regulator, it can use an external transistor to boost its 150mA rated current, it can incorporate a current limiter, it can be a shunt regulator, and there is even a circuit for its use as a switching regulator in the data sheet. To fully understand the 723 then is to fully understand low voltage linear regulators.

The datasheet's 723 suply with external pass transistor and current limit.
The datasheet’s 723 supply with external pass transistor and current limit.

On my bench there is a low voltage supply that is my go-to battery replacement when I am prototyping. It’s the supply I made using a 723 after reading the Art of Electronics power supply chapter all those years ago, and it is not unlike the circuit shown in figure 4 of the device data sheet.

A standard transformer, bridge rectifier and large capacitor produces an unregulated supply of about 14 volts. This is taken through a power transistor whose base is driven by the 723 output, and thence through a current sense resistor to the PSU output and the current sense line. A potentiometer lies across the output, whose wiper goes to the negative input of the error amplifier while the positive input comes from the reference. This feedback from the potentiometer sets the output voltage, which ranges from around 2 volts to just over 12 volts.

The 723 PSU, inside and out. Suddenly I'm a kid again.
The 723 PSU, inside and out. Suddenly I’m a kid again.

The magic part to me as a 19-year-old was the point at which I understood the current limiter. The sense resistor is a 1 ohm wire-wound component connected across the base and emitter of the current sense transistor, so when the current through the resistor reaches 600mA the voltage across it becomes enough to turn on the transistor. This in turn pulls down the base of the output transistor and limits the voltage to keep the current at the 600mA limit. Simple and straightforward as a grown-up, but as a rookie this was one of those lightbulb moments in which everything comes together and makes sense.

The supply itself is rather tatty, with my hand-written calibration on sticky luggage labels stuck to its front panel. Inside the box it’s a bit messy, with a mixture of second-hand components and the 723 on a piece of stripboard. Decades later I’d make a far better job of it, but it has served me well for all that time and will no doubt continue to do so.

I don’t know whether other engineers have a favorite integrated circuit, or whether I’m alone in coming to the realization that I’d nominate the 723 as mine. It might seem an odd choice, given that it’s not a component I’ve used many times in my professional career. But for me the elegance of a circuit that provides such versatile access to so many functions is attractive, and that it does so with such a simple but clever design is ample reason to like it even if I rarely need to design a linear regulator. Perhaps I should drop a few of them on my next order, and explore some of its other configurations.

Who knows, perhaps you should too.

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