This chapter is from the book
This chapter is from the book
This chapter is from the book
3.8 Example Power Supply Designs
Let’s get right into some design with example voltage supplies that are commonly needed in embedded systems: 5.0V, 3.3V, and 12V (for robotics enthusiasts).
3.8.1 5V with the LM/UA7805
The "7805" is probably one of the most well-known voltage regulators on the planet. The most common supply voltage for hobbyist and simple embedded systems is 5.0V (although this is changing rapidly to 3.3V); moreover, the 5.0V supply is the single supply needed in most systems, so anyone who has even built a single board computer or an embedded system has probably used a 7805 or derivative. As noted there are a number of manufacturers that make 7805 regulators; take your pick, for now we will just assume we are going to use the Texas Instruments variation named the UA7805. Figure 3.52 shows an excerpt from the datasheet. The 7805 usually comes in a TO220 package, although it can be found from other manufacturers in a TO92 package like a transistor comes in. In any case, the 7805 is so ubiquitous you don’t even have to look at the datasheet to hook one up, but we might as well just for the exercise, so please go to the Texas Instruments website at http://www.ti.com and search for "UA7805", and download and view the datasheet. Or you can find it on the CD in
- CDROOT:\DATASHEETS\ua7805.pdf
Open the datasheet up either way and take a look at it; there is a lot of information. The beginning of a datasheet usually shows the product pictures and some of the package options, then come the DC/AC characteristics, next the application notes, and finally the mechanical data. Of course this isn’t set in stone and you will see variations, but in most cases, you will find all of these sections in any reputable company’s datasheet. Once you’re done looking, find the example applications and take a look at the "fixed-regulator" example. As you can see, the application notes show the regulator connected with a Cin of 0.33μF and a Cout of .1μF. This is their suggestion and the bare minimum design in most cases, so don’t think it written in stone. In general, I prefer to use a 1uF (monolithic ceramic) for the input capacitor Cin and place not one, but two capacitors on the output Cout1 = 1.0μF (tantalum) and Cout2 = 0.1μF (monolithic ceramic) as shown in Figure 3.53. The two output capacitors form a high frequency noise filter that filters two decades’ worth of frequencies.
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Figure 3.52 Datasheet for UA/LM7805.
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Figure 3.53 Example 7805 5V design with input and output bypass/filtering capacitors.
Now, if you review a datasheet and it explicitly says to use very specific values then do so, but on many voltage regulators, you are free to improve the input / output de-coupling. Okay, next let’s improve the design a little bit with some protection. When you are connecting your power supply up to components on your board or interfaces, you never know what can happen when interfacing to the real world. For example, what if there is something that charges up and then discharges into your supply? Or what if you short the outputs? These are the kinds of things you have to design defensively for. If you review the datasheets more for the TI UA7805, you will actually see some design suggestions for these events. In fact, the XGS uses them as well.
3.8.1.1 Output Polarity and Reverse Bias Protection
Referring to Figure 3.54, we see our voltage regulator with the addition of two diodes D1 and D2. Let’s start with D1: D1 protects against reverse biasing the regulator. For example, if for some reason the output’s voltage is higher than the input voltage then this could damage the regulator; therefore in this case the D1 starts to conduct and "shunts" the current over the regulator. The diode D2 is for output polarity reversal; that is, say that a load is connected to the regulator’s output that is more negative that the ground. This can cause damage as well; in that case the more negative voltage at the anode of D2 makes D2 conduct and once again the signal doesn’t make it back into the regulator.
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Figure 3.54 Adding protection diodes to the voltage regulator design.
3.8.1.2 Input Voltage and Heat Management
The next two things to consider when designing a voltage regulator circuit is the maximum input voltage that the regulator can take. For example, the datasheet states that the 7805 can work with an input from 7–25V. Although 25V is acceptable, the regulator would generate a huge of amount of heat, so there is no need to drive it that hard. Better to set the input to commonly found wall transformer voltages of 9–12V DC. Additionally, there is the concern of heat management. Showing thermal calculation and heat sinking is beyond the scope of this book, but the rule of thumb is this: If you can’t touch it, it’s too hot! Therefore, make sure you place a heat sink on your regulator.
A heat sink is nothing more than a piece of metal that makes contact with your device to increase the heat flow from the device into the ambient environment. There are a number of types that clip on or screw on, but this is the number one biggest mistake digital engineers make with power supply designs: They crack open a book and then look up a design for a voltage regulator, copy it, but then don’t realize the design needs to be heat sinked! And they put the design into use and the power system fluctuates, gets too hot, etc. and burns out prematurely. The original PlayStations had this problem; you turn them on and after a couple hours they would overheat. They did since the designers didn’t know any better, but you do, so make sure you heat sink your voltage regulators with a good size sink. Figure 3.55 shows some images of common TO220 package heat sinks.
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Figure 3.55 Heat sinks for TO220 packages.
3.8.2 12V with the LM/UA7812
If you refer to the datasheet for the 7805, it has all the data for the 12V variant, the UA7812 as well! This is very cool, so all you do is look up the differences that are pertinent. The most important one of course is that we surely can’t regulate 9V down to 12V! Hence, we need to start with a larger supply; referring to the datasheet we see that the 7812 needs from 14.5–30V, so I might pick a commonly found wall transformer supply of 18V or 24V. With this is mind, the design is identical to that of the 5V 7805 design; Figure 3.56 shows the final UA7812 design.
Now, one problem might be that you don’t want to have two wall transformers, but you want to have both a 5V and 12V supply. No problem; simply start with 18V DC, rectify it if necessary, and filter it with a capacitor if necessary. Then use it to feed the 12V supply, take the output of the 12V supply and feed that to the input of the 5V supply, and you have both supplies. The only thing to remember is that the 12V supply is feeding the 5V system, thus if you have a total of 1A of current feeding the 12V system and the 12V can output all of it, that doesn’t matter if the 5V system is sinking 600mA of it. Your 12V supply will only be able to supply your 12V portion of your design with the remainder, which is 400mA; thus consider that if you do feed your 5V from a 12V then part of the 12V current drive is going to disappear to power the 5V.
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Figure 3.56 12V voltage regulator design.
3.8.3 3.3V with the TI TPS76933DBVT
3.3V is becoming the new standard for embedded systems. Ironically, it’s already two generations old; there are already 1.8V and 1.5V systems, and 1.2V are starting to show up as well. However, most manufacturers are about 3–5 years away from completely phasing out 5.0V, so 3.3V is still on its climb, so we don’t have to worry too much about 1.8V and 1.5V yet as hobbyists. In any case, there are a lot of choices for 3.3V regulators from a lot of manufacturers, but I have used a lot of them and found the Texas Instruments TPS76933DBVT to be easy to use, cheap, and work well. You can find it online at http://www.ti.com as usual, or look at the datasheet located on the CD in
- CDROOT:\DATASHEETS\tps76933.pdf.
Figure 3.57 shows the device’s first datasheet page. Referring to the datasheet, the first thing you are going to notice is that this is what’s called a surface mount device (SMT or surface mount technology). That is, you can’t place it in a breadboard or experimenter board; you have to either solder directly to it or get special SMT-to-through-hole DIP (dual in-line package) converters. Not to worry though, the company below
has a whole line of surface-mount-to-DIP and -through-hole converters. Basically, they make circuit boards that you solder your SMT parts onto and then you use the circuit boards (very small) as if they were through hole parts! Figure 3.58 shows some of their products.
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Figure 3.57 Datasheet for the TI TPS76933DBVT 3.3V regulator.
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Figure 3.58 SMT to DIP converter PCBs.
Aside from the fact that the regulator is SMT, it’s a great little device. It has 5 pins, and comes in a SOT-23, which means there are a total of 5 pins/contacts: 3 on one side, 2 on the other; refer to the datasheet. The pins are defined as follows:
- IN—Input voltage 2–10V.
- OUT—Regulated 3.3V.
- /EN—Regulator enable (active LOW); tie this to ground for the regulator to operate.
- GND—System ground.
- NC—No connection.
The only thing different about this regulator is that it has an enable pin, which is good if you want to digitally control a power supply, and that it’s surface mount. Now, let’s design a power supply with it. Once again, we assume that the stage 1 of the power supply is already in place and we are being fed power from that, but it’s not regulated. However, we have a decision to make: We can either feed from the main lines, which could be at 9–12V DC (however, the 12V would exceed the datasheet spec), but that’s not an option. Thus, we have to consider feeding from a lower supply like the 5V regulated supply. Of course, if we were making just a 3.3V system then we would design it such that the input from the wall transformer was 6–9V DC rather than 9–12V DC, but let’s assume that we have already created a regulated 5V supply in the system and that’s what we are going to feed the 3.3V regulator.
3.8.3.1 Following the Datasheet
The datasheet for the TPS76933DBVT is much more strict than the 7805, especially when it comes to the bypass/stability capacitors. Now, since I always overdo things I am fine, but many engineers under-design, and this is a part that can get those kind of guys in trouble. For example, if you refer to the datasheet for the TPS76933DBVT on page 11, the suggested design for the regulator C1 is 1.0μF, which is fine, but the capacitor across Vout is rated to be a minimum of 4.7μF and it has an ESR spec (equivalent series resistance—remember that?). So not only do you have to use a larger capacitor, you have to make sure that the internal resistance of the capacitor meets the ESR spec. Now, don’t get too worried; I will tell you right now, first use a larger capacitor (the spec says bigger is better), so 10μF will do, Make sure it’s tantalum and I guarantee you will be fine; you can look at the ESR (if you can find it) for the cap you have, but chances are you simply have some in your parts box and have no idea who made them, so we hedge our bets by using the larger suggestion and also reading that the larger cap allows smaller ESR values. With all that in mind, Figure 3.59 shows our design for the 3.3V regulator. Notice that I still added the protection diodes and I added another capacitor Cout2 = 0.1μF (ceramic monolithic) for high frequency noise.
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Figure 3.59 3.3V power supply.
3.8.4 Multiple Voltage Systems
There are actually regulators that output multiple voltages; for example the Texas Instruments TPPM0110 is capable of outputting 3.3V and 1.8V from a 5.0V input, so you need to use a LM7805 maybe on the input to get your 5.0V, then you can use the TI TPM0110 to get your 3.3V and 1.8V supplies in one chip. You can find the TPPM0110 on TI’s site at
or on the CD located at
- CDROOT:\DATASHEETS\tppm0110.pdf
Furthermore, there are hundreds of regulator families and literally thousands of regulators; just peruse any manufacturer of regulator devices such as Texas Instrument’s, National Semiconductor, Fairchild, etc. and you will be overwhelmed with choices. Whichever way you choose to go, a multiple voltage supply system has to follow one rule: All supplies need a common ground or return path for the current from each supply.
With all that in mind, let’s take a look at one possible power supply design that handles everything. First, let’s assume that we are getting an input of 18V DC (so we don’t need bridge rectification or a filter cap), we need 12V, 5V, 3.3V, and we must use separate voltage regulators—Figure 3.60 shows the complete design for this.
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Figure 3.60 Complete multiple voltage design.