Hi Shaq,
TPS7B7701 is designed to be in car battery direct connected applications. For a 12V car battery system, during load dump, the battery voltage (As shown in the Figure 12 as Us) may go as high as over 65V. The two diode cycled in yellow servers as a voltage suppress circuit. If you consider the top one to be forward biased, the bottom one is reverse biased. During the load dump, if the voltage at input pin is higher than the break-down voltage of the reversed biased diode, the two diodes will create a low impedance path to discharge the voltage. This will help to clamp the load dump voltage to be less than 45V which is the absolute maximum voltage rating for the input pin.
The diode at output pin is inductive AC-clamp. This clamp is critical for LDOs that power inductive load. Consider a situation that the load of the LDO is suddenly removed, due to the inductance at the load, the output pin is likely to see a voltage ringing with current dropping. The ringing could create a voltage lower than -0.3V which is the absolute minimum rating at output pin. The clamp will help to clamp the ringing within -0.3V to prevent the pin being damaged.
Here is an application note which also has these topics covered. It also covers common questions on automotive LDOs.
http://www.ti.com/lit/ml/slyb232/slyb232.pdf
Regards,
Jason Song
