Project Specification
Project Overview
USB-C powered low-power consumer environmental sensor node with integrated 2.4 GHz Wi-Fi (802.11 b/g/n) and Bluetooth Low Energy 5.x. The node measures ambient temperature and relative humidity using a digital sensor and is intended for fixed 5 V USB-C powered deployment.
Requirements
- USB-C receptacle input, sink/device role, default 5 V operation only
- Operates from USB-C sources advertising default current up to 3 A without USB PD negotiation
- Include input protection strategy covering reverse current/blocking behavior, over-voltage protection, under-voltage lockout, and over-current limiting/protection considerations
- Low-power Wi-Fi + BLE capable MCU/module
- Digital temperature/relative humidity sensor on a low-power digital bus
- 3.3 V system rail for MCU and sensor
- Consumer-oriented design with simple programming/debug access over USB
Architecture
- MCU/Radio: ESP32-C3-MINI-1 module (single-chip MCU + 2.4 GHz Wi-Fi + BLE 5, native USB, low-power capable, module avoids custom RF design)
- Sensor: Sensirion SHT40 digital temperature/relative humidity sensor on I2C
- USB-C power entry: USB-C receptacle configured as a sink using dual 5.1 kΩ CC pull-down resistors
- Input protection path: USB VBUS -> resettable fuse / current limiter stage -> surge/ESD protection -> reverse-blocking / ideal-diode style power-path controller stage with UVLO/OVLO control -> 3.3 V regulator
- Main regulator: 3.3 V low-IQ LDO sized for ESP32-C3 Wi-Fi burst current with appropriate thermal margin
- Programming/debug: Native USB D+/D- routed to ESP32-C3 USB pins with ESD protection
- User interface: power/status LED optional; keep base design minimal and low-power
Preliminary Power Budget
Assumptions for sizing:
- ESP32-C3 average active current: tens of mA, with Wi-Fi TX/RX bursts well above 300 mA
- Design peak budget target on 3.3 V rail: >= 500 mA
- SHT40 current is negligible relative to radio load
- USB-C source may be capable of 0.5 A to 3 A; design input path should safely accept compliant 5 V sources across that range without drawing more than required by the load
Why 3.3 V LDO is acceptable:
- 5 V to 3.3 V drop is 1.7 V
- At 300 mA average heavy activity, dissipation is about 0.51 W, which is manageable only with an adequately rated package and copper area
- At burst conditions the average thermal load remains much lower than peak instantaneous current, but regulator transient response and output capacitance must support radio current pulses
Protection Intent
- Reverse / backfeed protection: prevent system rail from driving current back into VBUS when unplugged or faulted
- OVP / surge protection: clamp ESD/transients on VBUS and USB data lines; add front-end device with input over-voltage protection or controlled cutoff if supported by chosen part
- UVLO: avoid brownout operation from drooping VBUS/cable conditions by selecting front-end device or regulator enable thresholding
- OCP: fuse/current-limiter stage sized from the system power budget; protect against shorts while tolerating inrush and radio burst current
Key Nets Planned
- VBUS_RAW
- VBUS_PROTECTED
- 3V3
- GND
- USB_D+
- USB_D-
- USB_CC1
- USB_CC2
- I2C_SCL
- I2C_SDA
- EN
- GPIO9_BOOT (or equivalent boot pin net)
Open Implementation Notes
- Prefer module-based ESP32 design to simplify RF compliance and antenna matching
- Keep ESP32 antenna region free of copper/components in layout
- One pair of I2C pull-ups for the whole bus
- Native USB avoids external USB-UART bridge and reduces standby current
- Verify all selected protection parts against 5 V-only USB-C sink use case and consumer BOM availability