7.3.10 Tracking Start-up and Shutdown

The TPS51200 also supports tracking start-up and shutdown when the EN pin is tied directly to the system bus and not used to turn on or turn off the device. During tracking start-up, VO follows REFOUT once REFIN voltage is greater than 0.39 V. REFIN follows the rise of VDDQ rail through a voltage divider. The typical soft-start time (t_SS) for the VDDQ rail is approximately 3 ms, however it may vary depending on the system configuration. The soft-start time of the VO output no longer depends on the OCL setting, but it is a function of the soft-start time of the VDDQ rail. PGOOD is asserted 2 ms after V_VO is within ±20% of REFOUT. During tracking shutdown, the VO pin voltage falls following REFOUT until REFOUT reaches 0.37 V. When REFOUT falls below 0.37 V, the internal discharge MOSFETs turn on and quickly discharge both REFOUT and VO to GND. PGOOD is deasserted when VO is beyond the ±20% range of REFOUT. Figure 20 shows the typical timing diagram for an application that uses tracking start-up and shutdown.

Figure 19. Typical Timing Diagram for S3 and Pseudo-S5 Support

Figure 20. Typical Timing Diagram of Tracking Start-up and Shutdown