USB Lithium Ion charger and Backup Power Supply circuit

A good power supply source is essential for microcontroller-based project. Having a Uninterruptible Power Supply backup circuit is essential for microcontroller projects where we need constant power source without any cut-out. This post describes the design of a simple USB Lithium Ion charger which forms part of a 3.3 V power supply circuit. Whenever mains supply fails, the lithium ion battery takes the load without any delay and when mains supply is restored, the battery goes to charging mode again.
USB Battery Charger

Lithium Ion Charger Circuit Design

The core of this circuit is the MCP73812T IC from Microchip. This IC is a linear charge management controller that provides specific charge algorithm for single cell Li-ion batteries. The IC can be powered by a single 5V which makes it perfect for a USB powered charger. The charging current is set by a single external resistor. The circuit presented here is a USB-powered Lithium ion charger combined in a backup powered supply.
Lithium Ion charger and Backup Power Supply circuit schematic

The circuit is powered at 5V through a micro USB connector. When the USB is powered, the FET Q1 is turned off and current flows to the Lithium Ion charger IC and to the 3.3V regulator (MCP1603). Under USB power, the Lithium Ion battery charges. When the USB power is removed, the FET Q1 turns on and current flows from the battery through the Drain-to-Source of Q1 into the 3.3V regulator MCP1603.

Components list

Lithium ion charger Parts list
A 3D model of the circuit is shown below:
battery charger 3D model

Assembled Circuit

Battery Charger PCB

The circuit was assembled and connected to a PC through a USB cable. The green LED indicates that the board is being powered by USB. The Lithium-ion battery was also connected and left to charge. A stable voltage of 3.3 V was measured at the circuit’s output. When the USB cable is disconnected, the LED goes off and the 3.3 V at the output is maintained.

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