Voltage and Current Sensing Circuitry

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The above circuitry allows the microcontroller to monitor the voltage and current at the input and output of the buck converter. This makes four sensors in total: Voltage Input, Current Input, Voltage Output, and Current Output.

Each sensor runs through a low pass filter (LPF). The filters are composed of two resistors and a capacitor. The LPF serves two functions. First, the resistor ladder scales the input voltage to limit that is acceptable to the microcontrollers analog to digital converter (ADC). Second, the filter cuts out high 'noise' frequencies. The frequency resonse of this filter can be calculated from the following equation:


H = \left | \frac{Vo}{Vi} \right | = \frac{R_2}{\sqrt{(2 \pi f C R_1 R_2)^2+(R_1 + R_2)^2}}


On the actual board, these components are laid out to accept parts in a 1206 package. This makes the large capacitances called out in the above picture (17.7uF and 20uF) unrealistic. Instead a 1uF capacitor is used. This gives the sensors a frequency cut-off of approximately 300 Hz. Additional digital filtering via software is expected within the microcontroller.