TPS54424: SW pin shorted to GND when Cboot is lost

Redding Pan 

First - Is the SW to GND short observed when the TPS54424 is unpowered or only when the TPS54424 is powered?

Second - did you try only replacing the Cboot capacitor on the second TPS54424 before replacing the TPS54424 to see if the SW to GND of the TPS54424 was damaged, or just forcing the low-side FET into an ON state?

Regarding how a missing Cboot could result in Low-side FET or SW to GND damage:

The BOOT to SWITCH "Bootstrap" capacitor powers the high-side driver and some supporting circuitry that operates between SW and BOOT when the SW voltage is raised due to the turn-on of the high-side FET.  Without the BOOT to SW capacitor, the high-side FET driver and its support circuitry do not function correctly.  Improper function of the high-side driver can result in turn-on of the low-side FET while the high-side FET is still in an "ON" state, and the resulting cross-conduction could cause damage to the low-side FET.

Since BOOT is internally powered by the BOOT Charge circuit, the internal driver can pull the high-side FET gate even without a BOOT to SW capacitor, but without a capacitor to raise BOOT with SW, SW will only rise to about 1V lower than BOOT charge voltage, collapsing the BOOT to SW voltage

Typical mechanisms for this include:

1) BOOT to SW powered Anti-cross conduction circuitry unable to detect High-side ON, allowing Low-side to turn-on before High-side turns off.

Internal anti-cross conduction circuitry detects the ON or OFF state of the high-side FET and inhibits the turn-on of the low-side FET while the high-side FET is on.  WIth the low BOOT to SW voltage, this circuitry may not detect the state og the high-side FET and properly level shift it to the ground referenced control circuitry for the low-side FET, allowing the Low-Side FET to turn on while the high-side FET is still one.

Cross-conduction at the start of low-side FET on-time can also induce sever negative voltage on the SW pin due to parasitic ringing.  This ringing may cause damage to internal SW to GND circuits to create a short from SW to GND that would typically be weaker (higher resistance) than a MOSFET short.

2) Low BOOT to SW voltage reduced over-drive of High-side driver components, delaying high-side Turn-off

Driver Propagation delay and turn-on / turn-off strengths are dependent on BOOT to SW voltage.  Low BOOT to SW voltage will increase delays and slow-down the turn-off of the high-side FET.  This could allow the low-side FET to turn-on before the high-side FET is off, inducing cross-conduction at the low-side FET turn-on, with similar effects to those described above.

3) Weak high-side FET pull-down with negative inductor current can also induce cross-condition

If the inductor current is negative or very low, the switching node can remain high even after the high-side FET turns off.  When that happens, there is a very rapid increase in the Drain-source voltage, which can capacitively turn-on the high-side FET if it does not have a strong pull-down.  This is commonly called dV/dt turn-on.  While it is most common on the low-side FET during the turn-on of the high-side FET it is also possible on the high-side FET during the turn-off of the low-side FET.

Determining whether the low-side FET has been damaged by cross-conduction, or other SW to GND circuitry has been damaged by negative voltage ringing would determine examination of the failed devices.