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One of the most basic sub-circuits for a project is the power supply. How power is supplied to a device, 120VAC, batteries, or dc wall supply is one of the first questions facing a design engineer with a new project. Then how to supply each of the various dc voltage levels is the next hurdle. There are various tools available, including linear regulators and switching regulators which then break down into boost converters and buck converters.
The linear regulator is the backbone of the dc world and you will find at least one on almost every circuit board you use. If you have a 9-volt battery, for example, and need 5 Vdc and 3.3 Vdc you could use two linear regulators to lower the voltage to the required levels. This is a very common practice, but it does have some disadvantages. The cost of dropping the voltage from 9 to 3.3 volts is efficiency. Some of the power from the battery will be turned into heat. For example, if you draw 20mA from the 9-volt battery, and you drop the voltage from 9 to 3.3 (a voltage drop of 5.7 V), you are generating 114 milliwatts of heat (P = V x I = 5.7 x 0.02 = 0.114). Whether this is an acceptable loss is a design decision.
When selecting a regulator, you want to select one that has a current rating close to the maximum current required by the circuit. The reason you don’t select, say a 5A regulator and then run 200mA through it is stability. Regulators that are very lightly loaded have a tendency to be less stable than those that run closer to the device specification. Cost and size will play a role in selection as well. A properly sized device will usually cost less and have a smaller footprint than an oversized device.
The power of a linear regulator is its ability to give a steady output voltage with varying input voltage. If you are running off of batteries the output voltage will remain constant even as the voltage of the batteries drops. That is until you hit a voltage known as the dropout voltage. This is the voltage where the output voltage begins to track the input voltage. It is not uncommon to have a dropout voltage 1 or 2 volts above your output voltage. So if you are using a 9-volt battery to regulate to 3.3V, with a 1-volt dropout voltage, then when the battery voltage gets to 4.3 volts, the output will begin to drop as the battery voltage continues to drop. Some of the new low dropout voltage (LDO) regulators have voltage dropouts as low as 80mV. That is a distinct advantage when you are concerned about battery life.
When you are going to make 10’s of thousands of devices, how many components you need and how much each one costs become very important. Linear regulators require very few external components compared to a switching regulator, but they are still important. Having a good input capacitor(s) will help reduce high frequency noise as well as ripple. The output capacitor may have requirements to keep the regulator stable. For example, parameters like the ESR (equivalent series resistance) of the capacitor can seriously affect the regulators performance. The datasheets usually provide good advice on component selection.
While devices are getting smaller and smaller they still have an upper limit on the operating temperature, junction temperature. Some thought should go into providing a pathway for excess heat to be channeled away from the regulator. This can be a complicated heatsink and forced air solution or as simple as an area of copper on the top and bottom of the board stitched together with vias. You can spend a good deal of time analyzing the thermal characteristics of your board. There are times when a small two-layer board to test out your design is the fastest and most economical path to a solution.
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