PCB Design Tutorial

Circuit for Signal Logic

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The yellow boxes highlight the circuits that were to be implemented on our PCB so that the MSP432 micro-controller could communicate with the single-board computer and the motors.

Power Requirements

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Current and voltage ratings were obtained from the links below. The max current value for the XL-320 was obtained experimentally as 0.45A, but 0.6A was used for a safety margin.

XL-320: Datasheet

AX-12: Datasheet

MX-28: Datasheet

MX-64: Datasheet

MX-106: Datasheet

Voltage Regulation

Linear regulators are easy to use since they often only require two external resistors and two capacitors to set the output voltage, however they are very inefficient in terms of efficiency. Linear regulators typically have efficiencies in the fortieth percentile range. They take the difference between the input voltage and output voltage and dissipate that energy as heat. This not only wastes power, but it also can generate heat, which, if not removed, can damage the board.

Switching regulators solve this efficiency setback with the tradeoff of a more complicated external circuit. Switching regulators typically have efficiencies in the eightieth percentile range. They achieve this by taking moving energy from the input to the output, bit by bit. This is done with a switch and controller.

Before selecting electrical components, I highly recommend looking at the PCB guide provided by Dr. Pachowicz (http://ece.gmu.edu/~ppach/ECE492-3/Resources/Seminar-PCB.pdf).

The regulator that was selected was the RT7258. This regulator was selected because out of the recommended chip packages that are easily surface mounted by hand, it provided the most current (8 amps). The datasheet can be found here: http://www.richtek.com/assets/product_file/RT7258/DS7258-01.pdf .

Having selected the regulator, the necessary quantity of regulators can be calculated from the current requirements displayed in fig. (1):

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In addition to the quantities seen above, two regulators will be needed to supply a 3.3V and 5V rail for the single board computer and micro-controller.

Setting the Output Voltage

The above circuit is for a 3.3V output, but the output can be adjusted by using different values for R1 and R2:

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For selecting resistors, it is good practice to choose a tolerance of 1%. Standard 1% resistor values can be found here: http://www.rfcafe.com/references/electrical/resistor-values.htm.

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