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As a novice to EE concepts who's trying to apply it to Arduino projects, I was doing hours of research on how to electronically switch another higher current (or voltage) device/component/subsystem on and off.

I've read many long questions on lots of forums trying to solve this same problem, but only see different configurations of MOSFETS or transistors or combinations with some resistors recommended as the solutions.

Only this tutorial mentioned another option which was load switches. I then found several articles (example, example, and example) extolling their virtues. They small, cheap, some are crazy efficient, and they seem way simpler (especially to someone learning the basics) with fewer (or no) extra resistor sizing and other ancilliary components needed. Appears to just be Vin, Vout, GND, Signal.

The very tricky knots people are trying to unravel with for example Arduino circuits with unexpected current flows down controls pins and being unable to saturate the transistors or wrong sizing the resistors seems way overcomplicated in many cases if these are as simple as advertised.

So what's the catch?

These articles read almost like marketing material.

  • Main Question: Why are standalone MOSFETS (and transistors) preferred over IC load switches?
  • What's the downside of load switches compared to transistors/MOSFETS?
  • Why aren't they as preferred or recommended?

I am surprised not to see them on the common maker movement beginner electronics stores.

JRE
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rfii
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    You can use a load switch if you want. I always use discrete mosfet's if I can because they are usually cheaper and they work well enough. Some load switches also have current limiting, so that can be nice, for example with USB. If you have to do anything more than just turn the power on and off, it may be better to use a load switch with extra features (such as current limiting). – user57037 Apr 15 '21 at 04:55
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    It's because they are expensive and probably there are no counterfeit examples made yet in PRC. It is cheaper to use generic transistors because of large availability. However you will find those smart switches in every industrial/ automotive product. – Marko Buršič Apr 15 '21 at 07:40
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    @MarkoBuršič They're not particularly expensive (at low current ratings, at least), especially factoring in their extra functionality. Worse for me is the lack of standardisation - if I can't get MOSFET X, there's a very good chance I can get a similar enough replacement MOSFET Y with the same footprint, not so easy with an IC. – awjlogan Apr 15 '21 at 07:57
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    "These articles read almost like marketing material" Yeah the "Sponsored by: Texas Instruments" part kind of gave that away. – Lundin Apr 15 '21 at 08:44
  • @rfii: MOSFETs are transistors. It is right there in the name: "metal–oxide–semiconductor field-effect transistor " You are probably trying to distinguish between MOSFETs and bipolar junction transistors (BJTs.) – JRE Apr 15 '21 at 08:58

2 Answers2

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Why are standalone MOSFETS (and transistors) preferred over IC load switches?

It depends on the application. If you have a "clean" environment without noise, conducted spikes/ESD, surges, potential short-circuits or polarity changes etc etc, then a pure MOSFET is simple and cheap.

If you have a tougher use-case scenario of the driver, then adding all the protection required will quickly get cumbersome and expensive. Plus you might need to adjust the gate voltage and polarity to suit whatever you are driving this from, like a 3V3 MCU, which often doesn't match the voltages of whatever you are driving.

One common scenario is when you use a high side driver with N-channel input. You'd then need a driver circuit as described here: https://electronics.stackexchange.com/a/188749/6102. Designing that whole driver circuitry manually is going to be delicate and expensive - these essentially come with a built-in voltage regulator.

What's the downside of load switches compared to transistors/MOSFETS?

You have to "marry" the silicon vendor. There are not much in the way of standards or second source for these kind of parts. Also, when it comes to certain kind of drivers, some manufacturers have almost monopoly over the market. Meaning they are free to give their customers the middle finger now and then since there's no competition.

Why aren't they as preferred or recommended?

You can't say that without a specific use-case in mind. If you look at cars for example, they almost exclusively use drivers over "raw" MOSFET. But if you have a use case where you just need to switch some internal load of your PCB, there's no reason to use a driver.

Lundin
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  • With the 2021 chip shortages, I see how "marrying" your design to one vendor's unique part can be a costly and risky decision. Generic replaceable parts allow you to adapt more easily if your primary vendor is out of stock or discontinues a product. – rfii Jul 29 '21 at 03:41
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IC load switches can become obsolete and limiting yourself to them limits you only the specifications they come in. Some things are simple enough that you can just DIY with more versatility and control.

Your "very tricky knots" are not very tricky at all when you consider that it is all beginners having those problems. That is not indication that it is a difficult issue; It is just indication that it is a beginner issue.

The only thing I really find attractive about a load switch is if it integrates the charge pump so I don't need to if I want the efficiency of a high-side NMOS. That takes more parts to do than it tends to be worth.

Also, marketing materials will always extol the virtues of whatever they are trying to sell you.

DKNguyen
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  • So the downside is only that they come in limited specifications (thereby limiting versatility/control) and are going obsolete as in won't be available? – rfii Apr 15 '21 at 04:38
  • @rfii Yeah. Sometimes you just don't want to rely on a part that is so task-specific when it is so simple to do yourself. – DKNguyen Apr 15 '21 at 04:39
  • OK I appreciate your response. Im surprised because some beginner electronics books I've read are many pages in teaching to calculate V, I, R over specific components and parallel/series circuits and don't even get to transistors or selecting components. So the simple selection of a transistor, mosfet, resistor for a transistor driven mosfet and the required calculations would be quite a bit more than the typical arduino project on popular sites like adafruit. Not saying it cant be learned but several hours of learning. A uni mechatronics course didn't mention transistors for over a week – rfii Apr 15 '21 at 04:49
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    @rfii It's because understanding transistors is not a beginner thing. If you wade through the actual transistor material then the simple things are no-brainers because you know what you can ignore and don't have to worry about. But if you teach those no-brainers to beginners something is liable to go wrong. Part selection is also an art since datasheets don't always provide the exact specifications you need or if they do, in the form you want them. – DKNguyen Apr 15 '21 at 04:51