The TPS2120 does not contain a CP2 pin. Instead, a select pin (SEL), enables override of the PR1 / VREF comparison. Once the voltage on SEL is greater than VREF, the device will select IN2 as the output. For manual switchover, an external signal can be connected to the SEL pin. For this example, the external MCU signal is a 3.3-V enable.

The TPS2121 can be configured for manual switchover in a similar manner as the TPS2120. Instead of a SEL pin, the 3.3-V external MCU signal can be connected to CP2. As long as the voltage on CP2 is higher than PR1, the device will select IN2 as the output. When the voltage on CP2 drops below PR1, the device will switch back to IN1. Therefore, the resistor divider on PR1 is configured the same as above, with the 5 kΩ and 10.2 kΩ.

For additional precautions, the voltage on PR1 can also be configured. If the voltage on IN1 were to drop, the device can automatically switchover to IN2. In this example, if voltage on IN1 drops below IN2 (3.3 V) then the device will switch to IN2. Therefore, the resistor divider on PR1 should be configured such that the voltage on PR1 will drop below VREF, when IN1 dips below 3.3 V. The bottom resistor is chosen to be 5 kΩ due to it's commonality and minimal current leakage. If a smaller leakage is desired, a larger resistor can be used. With this configuration, the top resistor was selected to be 10.2 kΩ. With this resistor configuration, the device will switch to IN2 when the voltage on IN1 dips to 3.22 V. Refer to Table 9-2 for additional information regarding the switchover configuration.

See Equation 5 for the VPR1 Calculation

Equation 5.