7. USB-C Power & Data Interface

7.1 ESD Protection Circuit

Electrostatic Discharge (ESD) protection is paramount in safeguarding sensitive USB interfaces from high-voltage transient events that can compromise device integrity. The STM32 WB55CEU microcontroller employs the ST USBLC6-2SC6 transient voltage suppression (TVS) diode to protect the USB D+ and D– lines against ESD and Electrical Fast Transients (EFT).

• Key Specifications:

  • Clamping Voltage: The USBLC6-2SC6 is characterized by a clamping voltage of 8.5V, ensuring that transient voltages do not exceed safe operating levels. This specification aligns with IEC 61000-4-2 Level 4 standards, which mandate protection against ±15 kV contact discharge.

  • Line Capacitance: With a typical line capacitance of 1.5 pF, the USBLC6-2SC6 minimizes signal integrity degradation. Low capacitance is crucial for preserving the high-speed data transmission quality inherent to USB protocols.

  • Bidirectional Protection: The device offers bidirectional protection for both VBUS (5V) and data lines, ensuring comprehensive safeguarding against transients irrespective of the discharge polarity.

• Placement:

  • Proximity to Connector: The USBLC6-2SC6 must be placed within 3 mm of the USB-C connector pins to ensure rapid response to transient events. This close placement minimizes the inductive path between the connector and the protection device, enhancing the efficacy of ESD suppression.

  • Ground Connection: A shared ground via with the connector shield is essential, maintaining an impedance of less than 10 mΩ. This low-impedance connection facilitates the swift diversion of transient energy to ground, thereby protecting the microcontroller and associated circuitry.

7.2 USB-PD Configuration via CC Pins

The USB Power Delivery (USB-PD) protocol is implemented through the Configuration Channel (CC) pins, which negotiate power roles and delivery parameters between connected devices. Proper configuration of the CC pins is essential for reliable power management and device interoperability.

• 5.1 kΩ Pull-Down Resistors:

  • Tolerance: Resistors such as the RNMF14FTC5K11 with a tolerance of ±1% are utilized to meet the USB Type-C 1.3 specification, which allows for a ±10% variance in resistor values. Precise resistor values ensure accurate detection of connected device roles and power profiles.

  • Function: These resistors provide a signal sink-only capability, establishing the default USB power mode at 5V with a current limit of 500 mA. This configuration is fundamental for devices that initially operate in a sink role before negotiating higher power delivery.

  • Layout: Symmetrical routing of the pull-down resistors is achieved through 10 mil trace widths and matched 0.5 mm trace lengths to the CC1 and CC2 pins. Symmetry ensures consistent electrical characteristics across both CC channels, facilitating accurate power role detection and negotiation.

• Leakage Current:

  • The pull-down resistors are designed to exhibit a leakage current of ≤1 µA at 5V. Minimizing leakage current is critical for maintaining efficiency, particularly in low-power modes, as excessive leakage can lead to unnecessary power drain and reduced battery life in portable applications.

7.3 Power Regulation with MIC5365 LDO

The MIC5365 Low Dropout Regulator (LDO) is employed to provide a stable 3.3V supply from the USB VBUS input, ensuring reliable power delivery to the microcontroller and peripheral components.

• Topology:

  • Input Voltage Range: The LDO operates with an input voltage ranging from 4.5V to 5.5V sourced from VBUS. This wide input range accommodates variations in USB power sources while maintaining consistent output.

  • Output Voltage: The regulator delivers a precise 3.3V ±2% output, with a maximum current capacity of 150 mA. This regulated supply is critical for the stable operation of digital and analog circuits within the microcontroller.

  • Dropout Voltage: At a load current of 150 mA, the MIC5365 exhibits a dropout voltage of 200 mV at a junction temperature of 25°C. The low dropout voltage ensures efficient regulation even when the input voltage approaches the output voltage, minimizing power loss and thermal dissipation.

• Decoupling Network:

  • Input Decoupling: A 4.7 µF X7R MLCC (0805, 10V, ±10% tolerance) is placed at the LDO’s input to provide bulk energy storage and stabilize the input voltage against transient fluctuations. This capacitor mitigates input noise and ensures a steady supply to the regulator.

  • Output Decoupling: A 1 µF X5R MLCC (0603, 6.3V, ±20% tolerance) is positioned at the LDO’s output to suppress load transients and maintain voltage stability. These capacitors are strategically placed within 2 mm of the LDO pins to minimize inductive and capacitive parasitics, enhancing the regulator’s transient response.

7.4 Thermal Management

Effective thermal management is essential to prevent overheating of power regulation components, ensuring both reliability and longevity of the PCB design.

• Power Dissipation Calculation:

  The power dissipation ($P_{\text{diss}}$) of the MIC5365 LDO is calculated using the formula:

  $$P_{\text{diss}} = (V_{\text{IN}} - V_{\text{OUT}}) \cdot I_{\text{OUT}} = (5 \, \text{V} - 3.3 \, \text{V}) \cdot 0.15 \, \text{A} = 0.255 \, \text{W}$$

  This dissipation results in a temperature rise ($\Delta T$) given by:

  $$\Delta T = P_{\text{diss}} \cdot \theta_{JA} = 0.255 \, \text{W} \cdot 220 \, \text{°C/W} = 56.1 \, \text{°C}$$

  For ambient temperatures exceeding 60°C, active cooling mechanisms such as airflow are recommended to maintain thermal stability and prevent thermal runaway conditions.

7.5 Layout Guidelines

Adherence to meticulous layout guidelines is crucial for ensuring the integrity of power delivery and data transmission through the USB-C interface.

• VBUS Routing:

  • Trace Width: VBUS is routed using 20 mil traces to accommodate a current capacity of up to 1A, ensuring minimal voltage drop and efficient power transmission.

  • Signal Isolation: Traces carrying VBUS are routed away from high-speed signal lines, maintaining a spacing of at least three times the trace width (≥60 mils) to prevent electromagnetic interference and signal integrity degradation.

• Grounding:

  • Star Topology: A star grounding approach is implemented, segregating analog grounds (LDO GND) and digital grounds. These domains are unified at the USB shield, ensuring a common reference point while minimizing ground loop interference.

  • Via Array: Under the LDO, a via array consisting of four 0.3 mm vias is employed to facilitate efficient thermal dissipation and provide a low-impedance ground connection. This configuration enhances thermal performance and maintains ground integrity under varying load conditions.

7.6 Validation Metrics

Validation of the USB-C power and data interface ensures compliance with design specifications and industry standards, guaranteeing reliable operation under diverse conditions.

• Voltage Ripple:

  The regulated 3.3V rail must exhibit a voltage ripple of ≤30 mV peak-to-peak (mVpp) within a 20 MHz bandwidth under a 100 mA load. This specification ensures stable power delivery to the microcontroller and peripherals, preventing operational instability due to voltage fluctuations.

• ESD Immunity:

  The USB interface must withstand ESD events as per IEC 61000-4-2 Level 4 standards, which include ±8 kV contact and ±15 kV air discharges. Successful adherence to these standards ensures robust protection against electrostatic and electromagnetic disturbances, safeguarding the microcontroller and connected devices.

• USB-PD Compliance:

  The Configuration Channel (CC) pin voltages must fall within the range of 0.25V to 0.61V when connected with a 5V VBUS. This compliance ensures proper USB-PD negotiation and reliable detection of connected device roles, facilitating seamless power delivery and data communication.

Conclusion

The USB-C power and data interface of the STM32 WB55CEU microcontroller is meticulously designed to deliver robust power management and high-fidelity data transmission. By integrating the ST USBLC6-2SC6 for comprehensive ESD protection, implementing precise USB-PD configurations via CC pins, and employing the MIC5365 LDO for stable voltage regulation, the design ensures reliable operation and compliance with USB-IF standards (TID 6200). Rigorous thermal management and adherence to stringent layout guidelines further enhance the interface's performance and durability. Comprehensive validation through voltage ripple measurements, ESD immunity testing, and USB-PD compliance checks guarantees that the USB-C interface maintains signal integrity and power delivery efficiency under transient and industrial environmental conditions, thereby underpinning the overall reliability and functionality of the Bluetooth-enabled PCB design.