9.2.4.2 Detailed Design Procedure

The primary constraints for this application are choosing the correct device variant for the monitored voltage and deciding the preferred solution to switch the back-up battery in and out of the system. For this design, the TPS3840PL30 provides an active-low, push-pull output topology that turns on the PFET when the 5-V rail monitored by VDD drops to 3.0 V. The diodes logically OR the power supply with the back-up battery and prevents reverse current leakage. Using this solution, the System Output remains above 1.8 V in all circumstances unless both the 5-V rail and back-up battery fail. The System Output voltage will follow the 5-V rail minus a diode drop until the 5-V rail drops to 3 V then the back-up battery switches into the system providing 3.3 V minus a diode drop to the System Output. When the 5-V rail comes back above 3.1 V accounting for hysteresis, the PFET turns off to disconnect the back-up battery from the system. Since this design disconnects the battery when not being used, this solution maximizes battery life.