1 Introduction

The BQ25856-Q1 provides automatic reverse mode operation for when the main power fails when auto reverse mode is enabled. As the input voltage, or VAC, falls below the set undervoltage threshold, the BQ25856-Q1 can automatically enter reverse operation and regulate the VAC voltage to a set voltage target. The BQ25856-Q1 also provides reverse blocking protection, adjustable charge voltage and charge current. The BQ25856-Q1 has a wide input range of 4.4V to 70V, a wide output voltage of up to 70V, and a maximum charge current of 20A.

There are two paths for the backup capacitors. The first path is to use electrolytic capacitors. Electrolytic capacitors can be charged to high voltages and store a decent amount of power cheaply in a small board size. The second path is to use supercapacitors. Supercapacitors have large amounts of capacitance, but a single-cell supercapacitor can have an operating voltage of around 2.1V to 3.5V. Using two supercapacitors in series to get a higher voltage can be necessary.

Figure 1-1 shows a simplified block diagram of the BQ25856-Q1 circuit.

Figure 1-1 BQ25856-Q1 Capacitor Backup Block Diagram