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Prototyping is a useful and powerful method in electronics which lets us analyze a circuit before using it in a system or turning it into a product. In this process we may need a single supply or multiple supplies to power the circuit depending on the type of the application. For example, an op-amp circuit may need a symmetrical supply such as +12V and -12V or a logic circuit may require both 5V and 3.3V at the same time. Some applications may need three or more. This means we should have a bench supply with multiple outputs or multiple bench supplies in the environment. This may not be always possible. This DIY Prototyping Board is designed to provide all the most used supply voltages that a designer will need during prototyping a circuit. The switching power supplies on the board output 3.3V, 5V, 12V and -12V rated at 1A independently. In addition those there are two precise voltage references at 5V and 2.5V provided especially for op-amp based applications.

Circuit Design

There are four independent DC-DC Step Down (Buck) Converters based on LM2675-ADJ which generate 3.3V, 5V, 12V and -12V at 1A. Since the adjustable type of LM2675 is used, the outputs depend on the feedback resistors. A simple formula is provided in the datasheet to calculate the output voltage. To be able to fine tune the output voltage, multiturn 100R variable resistors are used in the feedback line.

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To generate -12V, LM2675-ADJ is used in inverting configuration. The feedback resistors are chosen to generate +12V but in the circuit the output voltage and ground points are reversed in order to achieve a negative output voltage.

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Each converter can be enabled or disabled by using the switches connected to the on/off pin of LM2675 and the status of each line can be monitored by the help of the LEDs. In all the configurations when the on/off pin floats, the switcher is on. The switcher turns off when the on/off pin is connected to ground in the positive output configurations. In the negative output configuration, it should be connected to the negative voltage rail.

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The important components which determine the switcher performance namely input and output capacitors, inductors and diodes are carefully selected by following the design guide given in the LM2675 datasheet.

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In addition to the step-down converters, there are two precise voltage references on the board. REF5050 provides 5.0V from 12V input and AD780 provides 2.5V from 5V input. These are low noise, low drift and very high accuracy voltage references. They can be used in applications which require precise voltage reference inputs.

PCB Design

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The layout is important in switching power supply designs and special care has to be taken to achieve a good performance. The inductor, freewheeling diode, and especially the input capacitor should be placed as close as possible to the IC. They should be connected with heavy lines for the metal traces. There are almost four identical layouts used for each converter.

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Bill of Materials

Quantity Part Description Designator
4 LM2675M-ADJ 1A Step Down Voltage Regulator U1 U2 U3 U4
1 AD780 2.5V Precise Voltage Reference U5
1 REF5050 5V Precise Voltage Reference U6
4 15uF 50V Tantalum Capacitor X-Case C3 C6 C9 C13
2 100uF 10V Tantalum Capacitor D-Case C2 C5
2 100uf 16V Tantalum Capacitor D-Case C8 C12
5 10nF 50V Ceramic Capacitor 0805 C1 C4 C7 C10 C16
2 1uF 25V Ceramic Capacitor 0805 C14 C17
2 10uf 25V Ceramic Capacitor 0805 C15 C18
4 30BQ040 40V 3A Schottky Diode SMC D1 D2 D3 D4
1 Coilcraft MSS1260-333ML 330uH Power Inductor L1
3 Coilcraft MSS1260-473ML 470uH Power Inductor L2 L3 L4
4 1K %1 SMD Resistor 0805 R1 R5 R9 R14
1 1.8K %1 SMD Resistor 0805 R3
1 3.3K %1 SMD Resistor 0805 R7
2 9.5K %1 SMD Resistor 0805 R11 R13
2 0R SMD Resistor 0805 R6 R10
1 330R SMD Resistor 0805 R4
1 820R SMD Resistor 0805 R8
2 2.2K SMD Resistor 0805 R12 R15
4 100R Bourns 3296 Multiturn Potentiometer VR1 VR2 VR3 VR4
4 Red SMD LED 0805 LD1 LD2 LD3 LD4
4 Mini Slide Switch SW1 SW2 SW3 SW4
6 1×3 2.54mm Female Header TP1 TP2 TP3 TP4 TP5 TP6
1 DC-001 Power Jack J1
2 Mini Breadbord -

Assembly and Test

All the parts in the BOM list should be soldered except R6 and R10. Those 0R resistors should be soldered after testing the 5V and 12V lines because the voltage references are powered by those sources. All the capacitors should be soldered carefully without applying much heat for a long time. To get the precise voltage outputs we adjust the 100R multiturn potentiometers. Mini breadboards are mounted after soldering and testing all the circuits. Four 15mm spacers are mounted to the corner holes to protect the bottom side components.

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You can see a simple microcontroller circuit built on the prototyping board above.

Please feel free to ask your questions and share your comments by using the comments field below.