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Bench Talk for Design Engineers

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Bench Talk for Design Engineers | The Official Blog of Mouser Electronics

Don’t Overlook LDOs for Efficient Voltage Regulation Steven Keeping

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Climbers have several ways of reaching the peak of Snowdon, which at 1,085 meters is the highest point in Wales and offers views of five countries, 24 counties, 29 lakes and 17 islands from its summit. The hardy can choose several walking paths to the summit of which the easiest, but also the longest, is the seven kilometer Llanberis Path. The hardier still can traverse the Llanberis Path as part of the annual Snowdon Race. (You’ll need to post of time of less than 40 minutes if you hope to win the thing.) Unsurprisingly, most day-trippers head for the Snowdon Mountain Railway, a 7.6 kilometer, single-track, 800 millimeter gauge railway running alongside the Llanberis Path.

Running a train up an average gradient of 12.7 percent with sections as steep as 18.2 percent is a formidable mechanical engineering challenge. Conventional trains struggle to cope with gradients over two percent due to lack of adhesion between the driving wheels and rails. One popular solution is to limit the track gradient by zigzagging the line up the mountain. The downsides are dramatically increased civil engineering cost and a much longer journey for passengers.

With no precedent for such a steep railway in the United Kingdom, the Welsh engineers turned to the Swiss, the world experts in getting trains up mountains. The Swiss solution is as ingenious as it is simple; the train is pulled up the slope by attaching a cogwheel to the driven axle that engages with a toothed rack rail set between the running rails. Such a rack and pinion arrangement ensures that the driven wheels can’t slip no matter how severe the gradient. It’s a cheap, effective and low-maintenance solution that’s still working well over 120 years since it was first installed.

Beware the Trade-offs

Electronics engineers face technical challenges equally as tough as those overcome by the Snowdon railway builders. And the temptation is to look to the latest technology for the answer. One such example comes when building a power converter for a battery-powered device such as an IoT sensor or a wearable. A major consideration for the designer is to select a power converter that extends battery life so that maintenance technicians don’t have to swap out sensor cells too often or consumers aren’t inconvenienced by frequent smartwatch recharges.

Designers need a power converter to generate the required output voltage and current for product’s electronics from a given input power source. And it needs to do this during both steady-state and transient conditions. In an IoT sensor or wearable, the input power source is the battery, and the regulator needs to maintain the constant voltage demanded by the product’s electronics even as the cell discharges and its voltage tails off.

An obvious answer to this problem is a switching voltage converter, a technology that first appeared in the 1960s but continues to evolve today. When a switching voltage converter’s internal transistor is ‘on’ and conducting current, the voltage drop across its power path is minimal. When the transistor is ‘off’ and blocking high voltage, there is almost no current through its power path. Consequently, the transistor performs like an ideal switch and dissipates very little power-boosting efficiency of a switching regulator to over 90 percent in some applications.

But switching regulators, like zigzagging railway tracks, bring trade-offs. They are complex, requiring external feedback loops; they take up a lot of space because they require peripheral components such as energy storing inductors, and capacitors and resistors for filter circuits; they are expensive; and the transistor switching can generate electrical noise that can disturb the sensitive downstream silicon.      

Keep It Simple

An alternative is the low-dropout linear regulator (LDO), a device that traces its history back to an article written by Robert Dobkin (then a National Semiconductor engineer and later founder and CTO of Linear Technology) published in Electronic Design back in 1977. One key difference between the switching regulator and the linear regulator can be drawn from their respective names; where the former employs a switching transistor at its heart, the latter uses a transistor operating in its linear range.

In the linear regulator, the transistor works as a variable resistor operating in series with the output load. The regulator employs an integrated feedback loop using an error amplifier to sense the output voltage via a sampling resistor network, which is then compared with a reference voltage.

LDO regulators work in the same way as conventional linear voltage regulators, but feature an important tweak in their topology. Unlike conventional linear regulators, LDOs use an open-collector or drain topology enabling the minimum voltage drop from the input to output voltage—while still ensuring proper operation—to be as low as the saturation voltage across the transistor—plus a small safety overhead. 

As efficiency is high on the list of attributes for a power converter in a battery-powered device, care must be taken to make the most of this “low-dropout voltage”. LDOs always burn some power in order to regulate the output voltage and, because the device is essentially a variable resistor, its power dissipation is equal to the voltage difference across the device times its output current. Its efficiency is the ratio of output voltage/input voltage. Part of the engineer’s job then becomes to match battery input voltage as closely as possible to the desired output to make the most of the power budget. For example, for a given current and a battery input voltage of 3 volts, an LDO supplying 2.5 volts will be about 16 percent more efficient that one supplying 2 volts. Because of its topology, the difference between the input and output for an LDO will be lower than that of a linear regulator. If the engineer selects carefully, an LDO power converter can approach an efficiency of 90 percent, getting close to that of a switching regulator but without the complexity, expense and electrical noise challenges of that device.  

LDOs are less suitable for high current applications because the power dissipation climbs with the current for a given voltage drop. And the devices are unable to boost voltages. It is for these applications that the switching supply truly comes into its own. But for space-constrained, battery-powered products, the humble LDO, like the cog railway, is a simple and reliable solution to a challenging engineering problem.

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Steven Keeping gained a BEng (Hons.) degree at Brighton University, U.K., before working in the electronics divisions of Eurotherm and BOC for seven years. He then joined Electronic Production magazine and subsequently spent 13 years in senior editorial and publishing roles on electronics manufacturing, test, and design titles including What’s New in Electronics and Australian Electronics Engineering for Trinity Mirror, CMP and RBI in the U.K. and Australia. In 2006, Steven became a freelance journalist specializing in electronics. He is based in Sydney.

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