🔋 System Overview

The Mini USB UPS consists of the following main functional blocks:

1. Battery Charging Stage (BQ21040)

• Battery charger IC: Texas Instruments BQ21040.

• Configured for a 300 mA charge current, allowing slow charging of three 18650 Li-ion cells connected in parallel.

• Designed for continuous operation as a UPS, meaning the battery pack is normally under float charge conditions.

• Slow, controlled charging improves battery health and extends cycle life over time.

2. Battery Protection Stage (BQ29700)

• Protection IC: Texas Instruments BQ29700.

• Utilizes back-to-back N-channel MOSFETs for full cutoff protection.

• Provides comprehensive protection against:

  • Overvoltage (OVP)

  • Undervoltage (UVP)

  • Overcurrent (OCP)

  • Short-circuit events

3. Power Bypass / Automatic Switchover

• Implements a discrete power-path switching topology.

• When USB Type-C input power is present:

  • Input voltage is routed both to the charger and to the output boost stage.

• When USB input power is lost:

  • The bypass switches within a few microseconds (µs), connecting the battery pack directly to the boost converter, ensuring seamless transition and uninterrupted output.

• This provides UPS-like operation without a dedicated power-path IC, maintaining simplicity and efficiency.

4. Boost Converter Output Stage (TPS61089)

• Boost regulator IC: Texas Instruments TPS61089.

• High-efficiency synchronous boost design capable of delivering up to 3 A at the USB Type-A output.

• The converter is derated to 3 A to maintain a thermal balance and ensure reliable PCB operation.

• Provides a clean and stable 5 V output suitable for Raspberry Pi, SBCs, IoT nodes, and other USB-powered devices.

5. Additional Protections

• Resettable PPTC fuse for extra safety on both input and output lines.

• Built-in overcurrent and thermal protections in each IC stage.

⚙️ Key Features

• USB Type-C input and USB Type-A regulated output.

• Automatic switchover between input and battery with no noticeable voltage drop.

• High-efficiency boost stage with synchronous design.

• Battery health–oriented slow charging.

• Full protection for battery and load circuits.

• Compact PCB optimized for thermal and electrical performance.

📊 Typical Use Case

Designed primarily to:

• Keep Raspberry Pi or other single-board computers powered during power outages.

• Power microcontrollers, IoT gateways, and communication modules with continuous 5 V supply.

• Maintain operation of USB-powered embedded devices that must never reset or lose power.

🔬 Design Notes

• Battery pack: 3× 18650 cells in parallel (1S3P configuration).

• Typical battery capacity: ~7500–9000 mAh (depending on cell model).

• UPS designed for continuous connection; battery acts as backup reserve.

• Thermal management optimized in PCB copper planes and component placement.

• All simulation and validation tests performed using manufacturer reference models and load profiles representative of a Raspberry Pi 4.

📄 Documentation

• BQ21040 Datasheet

• BQ29700 Datasheet

• TPS61089 Datasheet

Additional resources:

• /hardware → schematics, layout, and Gerber files.

• /docs → thermal and functional analysis reports.

🚀 Project Status

• ✅ Schematic and PCB layout complete.

• ✅ Functional testing validated.

• 🔜 Long-term endurance and thermal stability testing in progress.

📷 Preview

(Add board photos or renders here)

📜 License

This project is released under the MIT License.
See the LICENSE file for details.