8.3.16 Pin-Fault Detection and Protection

The controller includes protection against most practical pin faults. These faults include open pins, pins shorted to adjacent pins, pins shorted to GND and pins shorted to VDD. The device performs pin fault checking at start-up, before the PWM is enabled. Table 5 summarizes the response to pin faults. Most faults cause either a latched shut-down, or failure to start-up. For UCC28634, all pin-faults are non-latching.

A short-circuit from the HV pin (pin 8) to the VDD pin (pin 6) is unlikely to occur, because pin 7 is not included in the package. The HV pin and tracking requires additional PCB spacing in any event to meet creepage requirements. However, if such a fault does occur, the device continues to charge the VDD capacitor through the HV pin external series resistor, and the power supply starts up and appears to operate normally. But because the HV and VDD pins are shorted, the internal HV current source cannot switch-out the external HV resistor, so it always dissipates power. This condition results in a large increase in no-load standby power. A 200-kΩ external HV resistor, dissipates 66 mW at 115 V_AC, and 265 mW at 230 V_AC. At load levels where the X-capacitor discharge function is operational, the short to VDD appears to be an AC-disconnect event, and causes the device to cycle on and off.