To prevent a sequencing violation, it is important to block reverse current and implement a disable signal to the PMIC. A Schottky diode can block reverse current when the input is removed. Additionally, capacitors can help maintain the input voltage level while the power-down sequence occurs. Depending on the system design, there are a couple ways to implement a disable signal.
For a system where the TPS65903x-Q1 is powered by the system input voltage, a supervisor can be used to create a logic signal, indicating if the power is at a good level. An example of this solution is shown in Figure 7-3.
An alternative solution is possible when a pre-regulator is present. In the case of the pre-regulator, the pre-regulator output capacitance can also act as the energy storage to maintain VCC1 for the necessary time. The total supply capacitance should be calculated to support the worst-case leakage current during power down so that the voltage is maintained until the power-down sequence completes. Figure 7-4 shows an example of this configuration.
To determine the capacitance needed at the output of the pre-regulator, use Equation 1. This equation is used to ensure that the power down sequence is complete before the device is disabled.