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Demonstration circuit 2118B is a combination step-down battery charger and step-up battery backup regulator using the LTC4040 monolithic system power manager. The DC2118B has an input voltage range from 4.8V to 5.5V with overvoltage protection (OVP) to 40V, as limited by the external MOSFET. The input current is monitored to prioritize the load up to the 2.5A threshold before charge current is reduced.

Battery Charging

As initially configured, DC2118B is programmed for 2A charge current limit when in charge mode. The charge voltage is jumper-selectable in 8 discrete steps from 3.45V to 3.6V for LiFePO4 chemistry or 3.95V to 4.1V for Li-ion chemistry, according to the F2/F1/F0 programming input settings.

The charge current limit is adjustable via the PROG pin resistor. End-of-charge status is triggered by current falling below C/8 and indicated by the CHRG LED. The optional NTC input provides temperature fault detection and status indicated by the FAULT LED. Charging can be enabled/disabled with the ENCHG jumper.

Battery Backup

Following an input power failure, the circuit automatically shifts to battery backup boost mode to regulate the VSYS output rail. The backup mode system output voltage is initially programmed for 5V. The input power failure status is indicated by the PFO LED.

If VSYS ever falls below the programmed reset threshold voltage, the RST LED will change states to indicate the need to reset the system and recharge the battery. Backup regulation can be enabled/disabled with the ENBST jumper.

For convenience, all LEDs can be disabled with the LED jumper to reduce the quiescent current of the system.

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