Introduction

Who?

• Reserved for @Daniel Puratich co-op

• @Kenny Na

• Context

  • COTS ELRS PWM Receiver: https://betafpv.com/products/elrs-micro-receiver

What?

Integrated PCB for:

• Buck converter for 3S Battery Input

• 5 V output at 4 A

• ELRS PWM Receiver

  • ESP8285 Backpack

  • SX128X or SX1280 RF IC

• 6x output connections for PWM for

  • 2x Flaperon

  • 1x Elevator

  • 1x Rudder

  • 2x Motors

• Single XT60 Battery Connection

• XT60 Battery Output intended to connect to ESC (electronic speed controller)

Why?

• Intended to integrate all electronics for the simple foam plane into a single PCBA.

  • ESC is still separate for the motor(s) but that project is coming later down the line.

  • 2024-07-28 Foam Plane Flight Test foam plane for context

• When using 12V->5V @ 5A Buck Converter Board we have two PCBAs

Overview

System Diagram

Open

At the current moment, the SX1281 (RF IC) and ESP8285 (central MCU) are built into an off-the-shelf package called the ELRS Micro Receiver from BetaFPV. In order to reduce size and weight of the fixed-wing aircraft, we want to integrate the buck converter/LDO regulator onto a single board with the MCU and RF IC. We also want 1 more PWM output (versus the Micro Receiver’s 5) for an additional point of control for the fixed-wing plane. We also want to allow for the possibility of connecting 2 motors, so the battery input will be directly split to 2 output XT60s.

This buck converter will be missing the reverse polarity protection from the Meghan’s board, as it was deemed not required for this project.

Prerequisites

Buck converter definition

Open

Top-down diagram (Phil’s Lab #60)

• DC-to-DC step-down converter; decreases voltage while increasing current

• More efficient than linear regulators due to the difference in voltage not being released as heat (“converted” to current)

• Components:

  • Source (Vin)

  • Switch

    • Usually a transistor (FET), rapidly switches on and off

      • The ratio at which the switch is on vs. off is called the duty cycle

    • When connected, it allows current to flow from the source to the inductor

  • Diode

    • Is activated when the switch is disconnected, due to the polarity of the inductor (L) flipping

  • Inductor

    • Stores energy in its magnetic field when switch is connected

    • Releases the energy when disconnected

  • Capacitor

    • Standard functions; reduces ripple in voltage and stores energy for a steady output

Basic operation

• Past the circuitry above, there is a control (feedback) mechanism monitoring the output voltage and adjusting the duty cycle to match the desired output.

• In addition, the diode in a switching regulator can be substituted for another switch (transistor); this is called a synchronous buck converter (versus a diode’s non-synchronous operation). Synchronous buck converters have better efficiency than their non-synchronous counterparts, due to diodes always having a voltage drop (typically from 0.3V to 0.7V according to Claude). Having another transistor switching removes this inefficiency, but also makes the internal circuitry of the IC more complicated.

A few calculations

The inductance (L) can be calculated based on the relationship between the voltage and current across the inductor. This relationship can be calculated with Equation (1):

V = L × dl/dt

Where the voltage across the inductor is V_IN - V_OUT, dI is the peak-to-peak I_L (∆I_L) (typically 10% to 60% of the maximum output current, I_OUT), and dt is Q1’s turn-on time, calculated with Equation (2):

dt = D × t_sw

With Equation (1), the state of the inductor’s energy storage when Q1 is turned on can be analyzed.

Input filters

• As frequency increases, capacitance decreases (until a certain inflection(?) point in real-life capacitors)

• As frequency increases, inductance increases (until a certain point in real-life inductors)

• When putting the same capacitors in parallel, their effects stack, pushing the inflection point further down the scale

• When putting different capacitors in parallel, their effects stack as well, creating high points of resonance in places where the capacitors are oscillating together additively

• …more within the presentation linked

ExpressLRS TX

What is a Backpack?

A Backpack is an add-on device that facilitates wireless communication between an ExpressLRS module and another device (e.g. a Video Receiver on a pair of FPV goggles) using the ESPnow protocol.

Simplex, Half/Full-Duplex transceivers

Simplex transceivers can only either: transmit or receive data. Half-duplex can both transmit and receive, but only one at a time. Full-duplex can perform both at the same time. For example, a portable radio might have a simplex receiver since it only needs to receive and output data, while a smartphone, which is constantly connected, would need a full-duplex transceiver for less latency and more bandwidth.

Crystal Oscillators

Explanation here…

Component Selection, Schematic Design

Buck Converter

Buck IC

• TPS564247DRLR Buck IC

  • Synchronous IC with up to 95% duty cycle

  • 3V to 16V input voltage

  • 0.6V to 7V output voltage

  • 1.2MHz switching frequency

  • Up to 4A of continuous current supported

On paper, this IC meets our design requirements. Calculations for verification begin below:

Calculating the duty cycle: Delta = ~(V_in)/(V_out,max x efficiency) = ~(5V)/(16V x 0.90) = ~0.3472…

Calculating inductor ripple current: Delta_I_L = ((V_in,max - V_out) x duty cycle)/(f_sw x L_average); where L_average is the average L value from the datasheet (L1_max - L1_min).

Delta_I_L with averaged L1 = ~((16V - 5V) x 0.347) / (1.2MHz x ((4.7 + 1.5)/2)uH) = 1.026A

Delta_I_L with “typical” L1 = ~((16V - 5V) x 0.347) / (1.2MHz x 1.8uH) = 1.767A

Calculating I_IC_max = high side current limit - Delta_I_L/2 = 6A - 1.767A/2 = 5.1165A

Our target output current of 4A is well under the calculated max IC current of 5.1165A.

Calculating peak switch/diode/inductor current: I_SW_max = I_out_max + Delta_I_L/2 = 4A + 1.767A/2 = 4.8835A

Inductor

Assuming the ripple current is 30% of the maximum output current:

L_min = (5V) x (16V - 5V) / (0.3 x 4A) x 1.2MHz x 16V = 0.00000238715… = 2.387uH

Since this is close enough to the “Typical L1” listed in the recommend components from the datasheet, the typical L1 value of 1.8uH will be selected for the application (which happens to be a common E-value, making sourcing easier).

Capacitors

Following the datasheet recommendation, a 10uF ceramic capacitor was selected from WARG’s existing library (GRM188R61E106KA73J) for the input capacitor. It comes in an 0603 package and is rated for 25V, which is well above the maximum input voltage of 16V. Another 100nF capacitor was selected as per the datasheet’s recommendation for high frequency filtering.

For the output capacitors, the datasheet states a typical C_out value of 44uF. To meet this spec, the design includes 2 22uF capacitors.

Bulk Capacitors

https://www.ti.com/lit/an/slyt670/slyt670.pdf?ts=1726655175328 (long and complex)

A 22uF aluminum radial capacitor was selected for bulk capacitance.

Feedback Resistors

5V = 0.6 x (1 + R1/10k)

R1 = 73.33k (ideal)

A 73.2k resistor was picked from the existing WARG library.

Connectors

• 1x XT60 Battery Connector (Receptacle)

  • When connected to the battery, this will provide voltage input for the aerial system.

  • The XT60 battery output is intended to connect to the ExpressLRS system and ESC.

  • The connector was added from WARG’s existing library.

ExpressLRS System

A quick and effective way to begin the design process is to view existing schematics of the SX1280 + ESP8285 online, as well as any example schematics from the datasheets. This will give a reference/guideline for how we want to design our own board.

Transceiver

• SX1281 RF IC

The IC series was pre-selected as per Daniel’s guideline of building based off of the BetaFPV ELRS Micro Receiver. Two options are available on DigiKey: SX1281 and SX1280. Since the SX1281 only adds an additional (not needed) feature and is cheaper on DigiKey, it was selected for this system.

Transceiver: Crystal Oscillator

• CS07103 (52MHz variant)

As per the datasheet’s recommended design, a 52MHz crystal from NDK America was picked. It is assumed that an amplifier is already integrated into the SX1281, which in conjunction with the crystal, creates an oscillator for the RF IC.

Microcontroller

• ESP8285 MCU

Going off of popular options and knowing what is integrated into the BetaFPV ELRS Micro Receiver, the ESP8285 was selected.

The ESP8285 datasheet says ; the VDD_RTC (device voltage for real-time clock?) is 1.1V or NC (no connection). Generic No ERC symbol is left on the pin. Every other “VDD…” pin is left connected to 3.3V as per datasheet guidelines of 2.7V - 3.6V.

TOUT for voltage sensing, implement? No.

The ESP8285 calls for a crystal oscillating between 24MHz and 52MHz. In order to maintain BOM simplicity, the CS07103 (52MHz variant) from above will be reused.

Connecting the SX1281 to the ESP8285 for SPI is something like this:

Do the same for all of the requisite pins.

Pull-up/down resistors

This is something usually covered in a first-year digital logic course/lab, but as a direct example: the SX_NSS net is connecting the NSS_CTS pin of the SX1281 to the MTDO pin (HSPI_CS; Chip Select for SPI) ESP8285. A separate pull-down resistor to ground is added, as when no data is flowing to the SX1281, we don’t want it to be floating (someone verify this statement). So, we add a resistor in series to ground, so that during normal operation, the signal is fine, and when the connection is off, any stray activity is pulled down to ground.

We can do similar for the NRESET pin (view pinout table from earlier). The SX1281 takes a signal from the ESP8285 to reset itself. This is pin is active low as stated in the table, so when it is off, it will be enabled. Since this means that the 8285 will send a low signal in order to trigger a reset, we want the pin to be pulled high until that occurs. So it’s the same deal as above, except we pull it to be powered by default.

Connectors

There are 3x6 2.54mm pitch pin connectors for connecting to the ESC. Each 6-pin line is for: 5V power, ground, and PWM output.

This is for current limiting, in case a user were to short the board.

• 1x U.FL Antenna Connector (Male Pin)

  • In order to facilitate flexible antenna positioning for the RF IC, the board requires a connector for the antenna. Also, Daniel would like to use this connector in his board.

  • The part, schematic symbol and footprint were created and uploaded to WARG’s library.

• 2.4GHz RF Chip Antenna (ESP8285)

  • The ESP8285 needs to communicate over WiFi, so the LNA_IN pin must be connected to an antenna, whether that be a trace antenna created during PCB layout or a chip antenna. For ease of design (avoiding the process and calculations of impedance matching), a chip antenna was opted for.

  • An existing chip antenna part from a previous board was placed from WARG’s library.

• XT60-PW-F Connectors (x2)

  • The fixed wing plane could be upgraded to two servos. For this, we want to deliver the buck converted output of 5V @ 4A to the devices. Motors at WARG often use this connector.

LED

A red LED was added to the GPIO pin on the ESP8285. In order to find the right resistor value, we can use the following formula: V_input - V_forward_voltage = desired_current x resistor. Say we wanted 10mA across the an LED with 2V forward voltage:

(3.3V - 2V) = 10mA x R.

This makes R equal to 130 ohms. We can find and place the appropriate resistor into the design.

Schematic Design Review

- Making sure each power net in the design has an appropriate label. Crucial for PCB layout time.

- A sort of review for terminology in EE

- Simulating the input filter to make sure the capacitor configuration is working harmoniously

Clean up schematic styling - following industry/WARG standards for a clean and readable final schematic

Next, can let EFS team know of the design and to review that the PWM outputs from the ESP8285’s pins are possible and appropriate. Once that is complete, an EE lead can do a final review, and PCB layout can begin.

PCB Layout

Buck Converter

The following are layout guidelines and examples from the TPS56424x buck IC datasheet.