In the TPS61094 with supercap solution, the MCU doesn’t need to control the TPS61094. TPS61094 can switch between buck charging mode and boost mode automatically. It can boost the supercap to power the high pulse load at data transmission and then charge the supercap during standby mode.

By setting EN = High and MODE = High, the TPS61094 is enabled to work in the auto buck or boost mode. TI suggests to set output target voltage (setting by R₁) is 3.3 V (> 3.6 V – 150 mV), which could help TPS61094 enter the buck charging mode automatically; set charging current to 5 mA, which could help get the maximum LiSOCl₂ capacity, according to Figure 1-2; set charging terminal voltage to about 2 V, which could help supercap get lower leakage current and longer working life time.

TI suggests to add a series resistor (R_in) of about 40 Ω between LiSOCl₂ battery and TPS61094 VIN pin. It could help limit the LiSOCl₂ battery discharge current during data transmission. The LiSOCl₂ battery discharge current is as

Equation 1.

The TPS61094 operation is as shown in the Table 3-1. During stand-by operation in the smart meter, because input voltage is higher than output voltage + 100mV, the TPS61094 enters auto buck mode. The bypass MOS turns on and NB-IoT is powered by LiSOCl₂. TPS61094 charges the supercap until it is fully charged. When the NB-IoT does the Rx / Tx transmission, there is a high pulse current at the output of TPS61094, because LiSOCl₂ can’t support high pulse current, the input voltage will drop. When TPS61094 detects the input voltage is lower than output voltage + 100mV, the boost mode actives automatically. So the supercap mainly supports the high load current.