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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:

Buck used to power Pixie from 12S.

V_in = ~55V

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

Based on price, current limit and efficiency, the LMR38020 is chosen.

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12V Buck

IC Selection

Based on price, current limit and efficiency, the LM5012 LMR38020 is chosen.

Datasheet

Typical Application Circuit

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• 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 300kHz400kHz, which should provide a reasonably low ripple current while minimizing switching losses

Frequency Setting Resistor

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For a 12V output and 300kHz switching frequency, R=100kFrom the table in the datasheet, for a 400kHz switching frequency, R=64.9K

https://www.digikey.ca/en/products/detail/yageo/RC0603FR-0764K9L/727344

Adjustable Output Resistor

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V_ref is given as 1V and V_out is 12V. Choose R_FBT to be 100K, then R_FBB=9.09K

https://www.digikey.ca/en/products/detail/yageo/RC0603FR-079K09L/727418

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

Output Inductor

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Choose standard value 150uH inductor:

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• Choose k=0.4 for current ripple

• Expected current draw from the load is 500mA, design for 750mA to be safe

• This gives L=67.1uH

• Choose closest standard value, 68uH

• 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/7447709151/1994053

• Note: the datasheet recommends using an inductor with a saturation current >4.5A to be well above the current limit of the buck converter. However, an inductor with the required inductance and high enough saturation current would be quite expensive. The converter supports 2.5A output, but since the load for this application is only ~500mA, the saturation current of 2.7A on the selected inductor should be sufficient.

Output Capacitor

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Setting the output ripple voltage to 50mV, C > 1.67 uF

The selected capacitor is 10uF rated for 35V. The voltage rating is kept 2-3x higher than the 12V that will be applied. The DC bias characteristics show that the capacitance remains above the required 1.67uF when 12V is applied.

7447709680/1638652

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 22uF for a 12V output

https://www.digikey.ca/en/products/detail/murata-electronics/GRM21BR61E226ME44K/4905534

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

https://www.digikey.ca/en/products/detail/yageo/CC1206KKX7R0BB225/5884563

• 100nF X7R rated at twice the input voltage is required for high frequency bypass

https://www.digikey.ca/en/products/detail/murata-electronics/GCJ21BR72A104KA01K/11618560

Bootstrap Capacitor

• 100nF 16V capacitor required to store energy for powering the mosfet gate drivers

https://www.digikey.ca/en/products/detail/murata-electronics/GCJ188R71E104KA12D/7363221

INA228 Current Sensing IC

Typical Application Circuit

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Shunt Resistor

• Resistance of 0.0005 ohms to minimize losses

• At max current of 75A, the resistor will dissipate 2.8W. To prevent overheating, a nonstandard resistor with a resistance of 0.0005 ohms and a power rating of 8W is selected

https://www.digikey.ca/en/products/detail/koa-speer-electronics-inc/PSL2NTEBL500F/1039674

Pullup Resistors x3

https://www.digikey.ca/en/products/detail/yageo/RC0805JR-0710KL/728241

Decoupling Capacitor

https://www.digikey.ca/en/products/detail/murata-electronics/GRM21BR6YA106KE43K/8323870

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GCJ188R71E104KA12D/7363221

Output Connector

• Output connector to Pixhawk with I2C current data from INA228 & 5V power from buck converter

https://www.digikey.ca/en/products/detail/molex/5024430670/2380429

Previous Research

Implementation Ideas

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