10.5.2.1 Shutdown Sequencing During Unexpected Power Loss

Using the adjustable Quick Output Discharge function of the TPS22810, adding a load switch to each power rail can be used to manage the power down sequencing in the event of an unexpected power loss (that is battery removal). To determine the QOD values for each load switch, first confirm the power down order of the device you wish to power sequence. Be sure to check if there are voltage or timing margins that must be maintained during power down. Next, consult Table 1 to determine appropriate C_L and R_QOD values for each power rail's load switch so that the load switches' fall times correspond to the order in which they need to be powered down. In the above example, we must have this power rail's fall time to be 4 ms. Using Equation 2, we can determine the appropriate R_QOD to achieve our desired fall time.

Since fall times are measured from 90% of V_OUT to 10% of V_OUT, using Equation 2, we get Equation 4 and Equation 5.

Equation 4.

Consulting Figure 6, R_PD at V_IN = 12 V is approximately 250 Ω. Using Equation 1, the required external QOD resistance can be calculated as shown in Equation 6 and Equation 7.

Figure 22 through Figure 25 are scope shots demonstrating an example of the QOD functionality when power is removed from the device (both ON and VIN are disconnected simultaneously). In the scope shots, the V_IN = 12 V and correspond to when R_QOD = 1000 Ω, R_QOD= 500 Ω, and QOD = VOUT with two values of C_L = 10 µF and 22 µF.

VIN = 12 V	CIN = 1 µF	CL = 10 µF

Figure 22. Fall Time t_F at V_IN = 12 V, R_QOD = 1000 Ω

VIN = 12 V	CIN = 1 µF	CL = 10 µF

Figure 24. t_F at V_IN = 12 V , QOD = V_OUT

VIN = 12 V	CIN = 1 µF	CL = 22 µF

Figure 26. t_F at V_IN = 12 V, R_QOD = 500 Ω

VIN = 12 V	CIN = 1 µF	CL = 10 µF

Figure 23. Fall Time t_F at V_IN = 12 V, R_QOD = 500 Ω

VIN = 12 V	CIN = 1 µF	CL = 22 µF

Figure 25. t_F at V_IN = 12 V, R_QOD = 1000 Ω

VIN = 12 V	CIN = 1 µF	CL = 22 µF

Figure 27. t_F at V_IN = 12 V, QOD = V_OUT