SLAU847D October 2022 – May 2024 MSPM0L1105 , MSPM0L1106 , MSPM0L1227 , MSPM0L1228 , MSPM0L1228-Q1 , MSPM0L1303 , MSPM0L1304 , MSPM0L1304-Q1 , MSPM0L1305 , MSPM0L1305-Q1 , MSPM0L1306 , MSPM0L1306-Q1 , MSPM0L1343 , MSPM0L1344 , MSPM0L1345 , MSPM0L1346 , MSPM0L2227 , MSPM0L2228 , MSPM0L2228-Q1
The VBAT PMU of the LFSS supports a charging circuit for an external supercapacitor connected to the VBAT supply pin. An overview of the charging circuit can be found in Figure 8-4.
The charging circuit contains an internal connect switch between the device VDD supply and the LFSS VBAT supply pin. The switch has an internal resistance and therefore limits the charge current. The supercapacitor needs to be connected with a series resistor to further limit the charge current. The external resistor needs to be in the same order as the internal switch resistor. This is important to start up the VBAT domain of the LFSS quickly.
The charging circuit is not active after power up of the device. The charging circuit needs to be enabled by SW.
For example, assume the charging circuit gets enabled with a completely depleted supercapacitor. In the initial state, the voltage over the capacitor is 0V, which means from modeling point of view this circuit node acts like a virtual ground. In this case, the internal and external current limiting resistor builds a voltage divider. If both resistors are the same value, the resulting voltage on VBAT is 1/2 of VDD. This is typically enough to start the operation of the LFSS.
The charging circuit contains an internal comparator circuit to supervise the external VDD supply. Should the external VDD supply drop below the VBAT supply, the charger switch will get disabled. This will prevent the reverse current flow from the supercapacitor back into the main supply. When the external VDD supply rises above the VBAT supply, the charging switch will be closed again.