4s-5V Buck Converter Design Rev 1

Owned by Kevin Li
Last updated: 2021-08-09
2 min read

Design Parameters

The specifications below govern the key decisions made for this revision:

  • Minimum Input Voltage = 12.8V

  • Maximum Input Voltage = 16.8V

  • Nominal Output Voltage = 5V

  • Maximum Output Current = 5A

The input voltage range of 12.8V - 16.8V was determined by the need to support the usage of a 4s Li-Po battery as a power supply. The output voltage was selected to be 5V due to its commonness. The maximum output current was chosen to be 5A based on the current draw of the following components:

  • Flight Controller

  • Servos

  • Sensors

With the recent intention of incorporating a NVIDIA Jetson Nano onto the drone for onboard CV, it is possible that the maximum output current must be rated higher (this is to be considered for Rev 2).

Design Calculations and Component Sizing

https://docs.google.com/spreadsheets/d/18mLThlfdizCUS8jiHNGjHdB69v7MyRbX1yp0kUL1zhw/edit?usp=sharing

Attached above is the spreadsheet containing all relevant design calculations and component sizing for this revision. The reference schematics from the datasheet of the selected buck converter IC and the TI WEBENCH Power Designer tool are also included. Listed below are the unique components sized:

  • Buck Converter IC

  • Output Inductor

  • Input and Output Capacitors

  • Feedback Resistors

  • Bootstrap Capacitor

Buck Converter IC

The buck converter IC was selected exclusively from Texas Instruments using their WEBENCH Power Designer tool. The tool takes in the input parameters listed below and generates a list of buck converter ICs with a basic circuit design included.

  • Supply Type (DC/AC)

  • Minimum Input Voltage (V)

  • Maximum Output Voltage (V)

  • Output Voltage (V)

  • Maximum Output Current (A)

  • Isolated Output? (Y/N)

  • Design Consideration Type (Balanced, Low Cost, High Efficiency, Small Footprint)

Shown below is the UI of the tool with the selected parameters for this design:

TI WEBENCH Power Designer UI

Data for the buck converter ICs of interest were then transferred to the design spreadsheet in the sheet “Buck Converter IC Selection”. By filtering through various parameters, the ideal buck converter IC could be selected. Factors such as the general complexity of the IC, switching frequency, efficiency, and component count were also considered.

Ultimately, the TPS565201DDCR IC of the TPS565201 series from Texas Instruments was selected. The device is designed to optimize external component count and to achieve low stand-by current. A few of the relevant details of the TPS565201DDCR IC are provided below:

  • Minimum Input Voltage = 4.5V

  • Maximum Input Voltage = 17V

  • Minimum Output Voltage = 0.76V

  • Maximum Output Voltage = 7V

  • Switching Frequency = 500kHz

  • Integrated FETs with 31mΩ and 16mΩ DCR

  • Estimated Efficiency at Maximum Output Current = 93.7%

  • Package: SOT-23-6 Thin

For more details, visit the TPS565201 datasheet.

Output Inductor

When selecting an output inductor, the following parameters are crucial to focus on:

  • Inductor Ripple Current

  • Inductor Peak Current

  • Inductor RMS Current

These three parameters are quantified by the following equations below. For the

 

 

The TPS565201 datasheet recommends an inductor with an inductance ranging between 1.0uH and 4.7uH. The reference schematic generated from TI WEBENCH Power Designer uses an inductor with an inductance of 3.3uH.