Low-current charger provides backup for USB-to-single-cell battery systems

introduction

This solution solves a very specific battery backup problem for low power applications. When the 12VDC power is lost, the user needs an integrated solution to provide charging and power switching for the backup battery. In particular, a path to disconnect the battery is needed when the voltage drops to 3.0V, not considering the fuel gauge management issue here.

It should be said that there are a large number of solutions on the market to solve the problem of load power supply and charging. However, the design discussed here is very easy to implement and can work independently without a microcontroller, with flexible design and low cost. This solution is implemented using analog circuits and does not require programming. It occupies a very small PCB area and can meet the requirements of space-constrained applications.

The design was inspired by radio frequency (RF) systems. In fact, this circuit is very useful for any low-power mobile device (for example, a device that periodically transmits a wireless signal). Most USB chargers support 4V-7V power supply, and inputs above 7V may damage the circuit. This design operates with input voltages below 7V and can withstand input voltages up to 28V to support USB power supply. The circuit uses USB power to charge a single lithium-ion (Li+) battery and provide backup power for low-power devices.

Design Specifications

This design assumes that a 12VDC power supply is used for charging. The circuit (Figure 1) includes a single-cell linear charger that charges a single-cell Li+ battery from a DC power supply. The power supply provides load power while charging the single-cell battery, and the circuit uses resistor current limiting. The maximum current is set by a resistor (50mA from -20°C to +50°C and 10mA from -40°C to +85°C), and the current is used to drive the load first. When the load does not require the full load current (400mA), the remaining current is used to charge the battery. Without DC power, when the battery voltage drops to a user-specified level, a voltage monitor can control an ultra-low resistance switch to disconnect the battery from the load. This operation can prevent deep discharge of the battery.

Figure 1

The package size of U1 is 2mm x 2mm, U2 is 1mm x 1mm, and U3 is 1.3mm x 1.7mm, so the circuit is very space-saving and only requires 5 external components.