Keep powering your system when the primary supply is intermittent: Page 4 of 6

April 12, 2019 //By Tony Armstrong and Steve Knoth
Keep powering your system when the primary supply is intermittent
In today’s world of constant connectivity, it is commonplace for many electronic systems to be always operational—regardless of their external environment or operating conditions. Said another way, any glitch in a system’s power supply, whether momentary, seconds, or even minutes, must be taken into account during its design process.

Thus, when external power is available, the device would operate as a step-down battery charger for single-cell Li-ion or LiFePO4 batteries while giving preference to the system load. However, if the input supply was to suddenly drop below the adjustable power-fail input (PFI) threshold the IC would need to operate as a step-up regulator, capable of delivering multiple amps to the system output from the backup battery. Accordingly, if a power failure were to occur, then the IC would need power path control to provide reverse blocking and a seamless switchover between input power and the backup power source.

Figure 1 shows a typical application schematic for this purpose using Analog Devices’ Power by Linear LTC4040 lithium ion battery backup manager.

Fig. 1: A backup supply using the LTC4040 with
a user set PFI threshold.

The LTC4040 also includes optional overvoltage protection (OVP) that protects the IC from input voltages greater than 60V with an external FET. Its adjustable input current limit function enables operation from a current limited source while prioritizing system load current over battery charge current. An external disconnect switch isolates the primary input supply from the system during backup. The LTC4040’s 2.5 A battery charger provides eight selectable charge voltages optimized for Li-ion and LiFePO4 batteries. The device also includes input current monitoring, an input power loss indicator, and a system power loss indicator.

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