The built in user power supplies provided by the Analog Discovery Module have limited current and fixed voltages. The fixed +/- 5 Volts and maybe up to 50 mA is OK for the simple Lab activities the students might do in their lab classes but to do larger more interesting projects, students may need more current. Similarly, the ADALM1000 entry level hardware module only provides fixed positive power supplies of 2.5 and 5 Volts. Unlike the Analog Discovery the ADALM1000 supplies can source ( or sink actually ) up to 200 mA. But having just +5 V and the lack of a negative voltage power supply can again limit the range of projects that can be explored using the internal power supplies. Professors often ask, “The built in power supplied are too limiting, we need our students to be able to make use of these teaching tools for multiple semesters if possible, and increasingly more sophisticated projects”. Possible external supply options for use with Analog Discovery were proposed in an earlier Blog entry. The suggestions proposed here, certainly could be used with Analog Discovery, are mainly targeted at providing a negative supply for use alongside the ADALM1000. In this earlier Blog I addressed the 0 to +5 V input range limitation of the ADALM1000 through the use of resistor dividers.
Do not take any of the following information an official endorsement of any of these products and these suggestions are provided as examples only.
A possible solution would be to use batteries to supply the needed negative supply voltage. Standard 9 volt batteries and connectors are inexpensive and readily available. Holders for multiple (from two to eight) 1.5 volt AA cells are also common and relatively inexpensive. The drawback with using conventional batteries is that they usually come in increments of 1.5 Volts, 1.5 ,3, 4.5 etc. and their voltage changes with changes in load and as the cells discharge. A possible solution to this would be to use a voltage regulator to produce a more common supply voltage like 5 V. A readily available battery powered 5 V supply can be had for $1.00 from the local Dollar Store. Shown in figure 1 are small plastic AA battery powered chargers with a USB port sold as Emergency Cell Phone Chargers, and a version of a USB charger that you plug into a 12 V car power outlet.
Figure 1 USB Cell Phone Chargers
Inside the plastic case of the AA battery version is a 3V to 5 V inductive boost DC-DC voltage converter much like various models that can be found on the Web, see figure 2. I’ve seen these on the web range in price from as much as $5 to as little as $1.
Figure 2, Example 3 V to 5 V boost regulator.
Inside the 12 V car USB charger is a 12 V to 5 V inductive buck DC-DC converter much like the one shown in figure 3.
Figure 3 Example 12 V to 5 V buck regulator
The nice thing about the AA battery chargers over the bear board regulators is the plastic case and AA battery holder. A battery holder for 2 AA cells with snap terminals such as the one shown in figure 4 can cost between $0.60 and $1.00. So having the battery holder built in is a bonus.
Figure 4, Two cell AA battery holder.
To access the 5 V output, the mini/micro connector end of a spare USB cable can be cut off and wires from the A connector end can be stripped back and a 0.1” header soldered on to plug into a solder-less breadboard. Or the USB connector can be removed from the PCB and wires or pins connected directly.
The extracted schematic for the 3V AA battery charger in figure 1 is shown in figure 5. The part marking on the 6 pin regulator IC was indecipherable but the pin-out seems to be the same as many similar devices from multiple suppliers. This is the textbook standard boost converter configuration. The converter runs at around 620 KHz and operates down to 2.2 V input voltage. It uses an adjustable regulator with an external feedback voltage divider, so this might be of use to improve performance or make a variable power supply. More on that to come.
Figure 5, 3V to 5V Boost converter schematic
The relevant part of the 12 V car adapter buck converter schematic is shown in figure 6. The part marking on the 8 pin regulator IC is AD84064 from a supplier in China ( this is not an ADI part number ). This is the textbook standard buck converter configuration. It uses a fixed +5 V regulator. This chip also provides a built in resistor voltage divider to produce a 2V bias for the D+/D- USB I/O pins and a pull up resistor to drive an LED ( pins 8 and 5 not shown in the schematic ). This buck converter seems to need a minimum input voltage of between 6.5 and 7 volts to operate. The maximum switching frequency was measured at around 400 KHz.
Figure 6, 12V to 5 V Buck converter schematic
Rather than use a 12 V battery I’ve found that this regulator works just as well with a standard 9 V battery. After removing the small PC board from the plastic case, as in figure 3, it is a simple matter to attach a 9V battery snap connector to the input voltage terminals. The two halves of the plastic shell could then be replaced inclosing the PCB for more protection.
Given the fact that either of these 5 V regulators is battery driven, the nodes we choose to be the common, 0 V, terminal and the “supply” voltage is totally arbitrary. So to use one of these as a negative supply we can choose to connect the positive output terminal to the common ground of our circuit (and the ground of the ADAML1000) and use the negative output as our supply voltage.
As an alternative to using batteries, input DC power for these DC-DC converters could come from a wall powered step down transformer unregulated power supply adapter ( so called “wall warts” ) but care will need to be taken to check that both the positive and negative output terminals are fully isolated from main power system ground.
Unlike linear regulators, these are switching regulators and there will be significant noise on the output especially when lightly loaded. Adding an LC pi filter in series with the output such as shown in figure 7 can greatly reduce the switching noise. Depending on the specific PCB layout it may be possible to open the feedback loop and re-close it at the output of the filter. This will eliminate any DC drop in voltage due to the internal resistance of the inductor. Including a resistor in series with the feedback can increase the output voltage if it happens to be slightly less than 5 Volts or you want an adjustable output that is higher than 5 V.
Figure 7 Adding an output filter network.
In addition to simply using such DC-DC converters as power supplies for experiments and design projects, they can serve as educational opportunities to study the inner workings of switch mode DC-DC converter circuits. The two ICs on these DC-DC PCBs are simple designs with not a lot of extra functionality There aren’t many alternate configurations of the Buck converter. Here is a more advanced converter configuration that works well and can be constructed around the IC on the boost PCB.
Split + and – 5V supplies can be generated from 3 volts ( the 2 AA batteries ) using a topology like the one shown in figure 8, based on the boost regulator and a Coilcraft HPH1-0190L 6 winding coupled inductor. This topology consists of an unregulated Ćuk converter tied to the same switching node as a regulated SEPIC converter. This combination results in two supplies that track each other very well under all but a 100% load mismatch. Depending on how fresh the AA batteries are, somewhere between 50 mA and 100 mA can be drawn from the +/- 5 V outputs.
Figure 8 Creating Split + and – 5V Rails from a Single 3V Input Voltage
To build this circuit from the boost regulator PCB you will need to remove the thru-hole mounted USB connector and 4.7 uH inductor. The surface mount devices, the IC, C1, C2, R1, R2 and D1 can all remain in place. You will need to cut the feedback trace on the PCB from D1 to the top of R1 (note figure 5). Removing the surface mount diode D1 will also accomplish the same thing. Inserting a variable resistor in series with R1 (or in place of R1) will provide an adjustable output voltage.
Coupled inductor L1 is three windings of the HPH1-0190L Hexapath inductor (available in the ADALP2000 analog parts kit). Be sure to follow the dot polarity on the windings. Added diodes D2 and D3 are 1N5819 Schottky diodes. C3 and C4 are 1uF capacitors, be sure to correctly connect if polarized capacitors are used. Output filter caps C5 and C6 can be just about any value greater than 10 uF for demonstration purposes.
As always I welcome comments and suggestions from the user community out there.