TIDUEM8B March   2019  – February 2021

 

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2.3.1.3 TPS3840 SVS

The TPS3840 device is an external supply voltage supervisor (SVS) that is used to externally reset the MSP432 MCU. The TPS3840 maintains very low quiescent current, which enables this device to still be used if there is a power outage and the meter is running from a backup battery. The MSP432 MCU has an internal SVS device that can be used as well, which will suffice for this application; however, using an external SVS instead of the internal SVS of the microcontroller adds an additional layer of security since it is not independent of the microcontroller, and therefore, is less affected by any issues that affect the microcontroller itself.

In this design, the TPS3840DL20 device variant is specifically used, which has a negative-voltage threshold voltage of 2 V. When the voltage rail that powers the MSP432 MCU drops below 2 V, the TPS3840 device resets the MSP432 MCU. When the monitored voltage rises above the undervoltage threshold plus hysteresis voltage value (approximately equal to 2.1 V total) , the RESET pin of the TPS3840 is pulled back high after a user-defined reset delay time, tD, elapses. tD is determined based on the value of the capacitor connected to the CT pin of the TPS3840 device. In this design, a 0.33-µF capacitor is connected to the CT pin of the TPS3840 device, which leads to a reset delay time of about 204 ms.

The TPS3840 device is available with both push-pull and open-drain outputs. The open-drain output is specifically selected for this design since a 47-kΩ pullup resistor is recommended in the JTAG circuitry of the MSP432 MCU.