Powering your prototypes: Linear or Switching regulators?

A common challenge among any electronic design is providing the right power to our devices. To make things more complex, each device might operate at a different voltage. For example we could have a LED lamp operating a 12V, an actuator demanding 5V, and our micro-controller operating at 3.3V. The most common approach is to have a voltage input providing the highest voltage and to lower it down to the different required voltages. We have two technologies at hand in order to achieve this:

  • Linear regulators
  • Switching regulators

Linear regulators are cheaper and simpler to design, but they are quite inefficient. Their working principle is to dissipate electrical energy into heat in order to lower the voltage. They provide a smoother and more stable voltage, but the voltage drops that they are able to achieve are smaller than the switching regulators.

Switching regulators are a little bit more complex to integrate in a design, as they required a few additional components and are a more expensive. In theory, they would generate no heat and no power would be lost. Nevertheless, when implemented with real materials they achieve efficiencies between 70% to 90% depending on the amount of current and voltage. For lower currents they are less efficient, and they might be even less efficient than a linear regulator for low voltages drops and small currents.

As linear regulators are simpler and cheaper, the rule of thumb is to first check if it can do the job, and if not, to consider the switching regulator.

As the working principle of a linear regulator is to lower the voltage by dissipating electrical energy into heat, we first need to compute how much power it will have to dissipate. Electrical power is computed as $P = V I$, and as the current wont change, the dissipated power can be computed as:

\[ \Delta P = V_{out} I_{out} – V_{in} I_{in} = (V_{out}-V_{in})I \]

For example, if we need to convert 5V into 3.3V and we require 500 mA of current, the it will dissipate: (5V – 3.3V)*500 mA = 0.85 Watts

Lets look for options in the digikey parametric search. We can go to the products section and look for “regulator:

https://www.digikey.com/products

And there we can see the category PMIC – Voltage Regulators – Linear 

We select all the voltages inputs greater than 5V and all the 3.3V voltage outputs:

Then we set the maximum expected current:

Finally, we want to make sure that they are available as Digi-Reel. The Digi-Reel option allows us to order lower quantities and still be able to automatize the manufacturing process with an assembly partner.

Finally, we order by price, and here check our winner:

In the datasheet we can see a typical application:

The next step is to check in the datasheet the thermal resistance:

The temperature at the juncture can be computed as:

\[
T_{juncture} = T_{ambient} + \theta_{JA} P_{dissipated}
\]

For a 25 degrees Celsius ambient temperature we get a juncture temperature of 172.05 degrees Celsius. In the datasheet we can also see that the maximum junction temperature is 150 degrees Celsius. This means that we will require to add a heat sink and empirically test if that would be enough. A heat sink will usually be able to reduce the $\theta_{JA}$ to half, so in this case it should be sufficient. This package is quite small, and thus it dissipate the heat worse than a bigger package. Another option is to look for a different package.

If we scroll down, we will see a bigger package:

In the datasheet we can see that this device is offered in many different packages:

With the TO-252 package now we get a junction temperature of 25°C + 0.85 Watt * 90°C / Watt = 101.5°C which is within the operation range of this voltage regulator.

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