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• The 12V output is left as a power pad so that a connector can be soldered onto it and a harness can be made as needed

• The Molex connector is based on the connector standard for the Pixhawk, shown below

• A 5V-3.3V LDO was added to provide a 3.3V rail for the SCL and SDA lines, as per the Pixhawk standard

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5V Buck

IC Selection

Ratings to keep in mind: Pixie ratings → Power Distribution Architecture

Buck used to power Pixie from 12S.

V_in = ~55V

I = ~3A (Pixie draws around 3A max) Requirements

• Steps down 18-55V to 5V

• Current limit should be at least 5A

IC Selection

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Based on simplicity and efficiency, the LMR38020 is chosen.TPS54560B is chosen.

Datasheet

Typical Application Circuit

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Pinout & Functions

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Switching Frequency

• Higher switching frequency allows for the use of smaller inductors and capacitors as well as lower output ripple, but it comes at the cost of increased switching losses

• Since this application involves a relatively high voltage input, the selected switching frequency is 400kHz, which should provide a reasonably low ripple current while minimizing switching losses

Frequency Setting Resistor

• For f_sw= 400kHz, R_T = 242K

• Choose closest standard value, 243K

https://www.digikey.ca/en/products/detail/yageo/RC0603FR-07243KL/727084

Output Inductor

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• V_{in_max} = 55V, V_out = 5V, I_out= 5A, K_ind = 0.3 (recommended by datasheet), and f_sw= 400kHz

• This gives L_min = 7.6uH. Choose the next standard value of 8.2uH. The saturation current of the inductor should be greater than the switch current limit of 7.5A

https://www.digikey.ca/en/products/detail/würth-elektronik/7443330820/2175573

Output Capacitor

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12V Buck

Requirements

• Steps down 18-55V to 12V

• The 12V rail is only used to power the vtx, which has a 500mA current draw. Ideally, the current limit of the buck converter should be closer to 750mA to be safe

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https://www.digikey.ca/en/products/detail/yageo/RC0603FR-07100KL/726889

Output Inductor

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• V_in = 55V, V_out = 12V, f_sw= 400kHz, I_out = 2A

• K = ΔI_L/I_out, choose k=0.4

• Using the above equation, L = 29.3uH

• Choose closest standard value, 68uH33uH

• Inductor saturation current should be >= I_sc , the high side switch current limit of 3.2A, to avoid component damage due to high current when the inductor saturates

https://www.digikey.ca/en/products/detail/würth-elektronik/7447709680744770133/16386521638634

Output Capacitor

• Output capacitor allows for the load to be powered during the off state of the transistor and accounts for load transients

• Voltage rating should be 2-3x the output voltage to account for voltage spikes

• Use the datasheet recommended value of The datasheet recommends 22uF for a 12V output

• Use 3x 22uF to account for DC bias at 12V

https://www.digikey.ca/en/products/detail/murata-electronics/GRM21BR61E226ME44KGRM32ER71E226KE15K/490553413904980

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Input Capacitor

• At least 4.7uF X7R rated at twice the input voltage is required to reduce ripple and maintain input voltage during load transients

• Option 1 ($4.29): https://www.digikey.ca/en/products/detail/murata-electronics/KCM55LR72A475KH01K/2785887

• Option 2 ($2.67): use 3x 2.2uF 100V in parallel

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