When the voltage on POTP drops below VPOVTH the conversion is stopped and the device enters fault.

The fault is managed as conditioned-restart.

8.7.7

Primary side undervoltage lockout

In case the voltage on PVCC drops below VPVCCOff the IC enters auto-restart fault mode.

8.7.8

Fault summary

The following table summarizes the faults and their effect.

Table 10. Faults table

Fault

Side

Effect

Comment

UVLO

Primary

Auto-restart

OVP

Primary

Enter burst

Primary side stops and enters burst. In case the secondary side gives

no signal, the system enters UVLO.

OCP

Primary

Auto-restart

Second level OC may happen in case of output short. The effect is a

shutdown.

SSTO

Primary

Auto-restart

Soft-start timeout (SS pin reached VSSEND).

POTP

Primary

Conditioned-restart

Overtemperature protection.

BO

Primary

Conditioned-restart

In case of brown-out, switching is stopped. Primary side stops until the

BO condition disappears.

All fault conditions are cleared.

XCD

Primary

Conditioned-restart

X-Cap discharge is managed in the same way as brown-out.

OCP

Secondary

Load switch

Turn off

If load switch is not present, protection is performed by primary side

OCP.

OVP

Secondary

Enter burst

If OVP is triggered secondary side stops and waits the voltage drop

under safe condition.

UVP

Secondary

Auto-restart

If UVP last for 480us the system stops switching waiting for primary side

restart.

UVLO

Secondary

Auto-restart

If secondary side hits UVLO, the system stops switching waiting for

primary side restart.

SOTP

Secondary

Auto-restart

The secondary overtemperature value is programmable and is

considered also a warning temperature (if enabled) before the SOTP.

The fault does not stop switching, but turns off the load switch and

sends a hard reset command on the USB-PD.

8.8

X-cap discharge

Safety regulations such as IEC 62368-1 require that the capacitors directly connected to the input plug (so called

“X-Caps”) are discharged when the converter mains connector is removed from the outlet, in order to avoid the

risk of electrical shock to the user.

Typically, this function is performed by means of a resistor in parallel, but this method cannot be applied in case

the converter requires very low power consumption during light-load or no-load operation, because the losses of

the X-cap discharging resistor would be too high.

The ST-ONEMP monitors the input voltage through the HV pin and discharges the X-capacitors through the HV

pin TXCDDET after a plug disconnection is detected.

The discharge continues until the voltage on HV falls below VHVmin or the plug is connected again.

Note that a DC voltage on the HV pin is interpreted as a plug disconnection, so the device cannot operate with

DC input voltage.

ST-ONEMP

X-cap discharge

DS13989 - Rev 2

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