The DIS/FLT pin is an input/output pin. It can be

• Externally driven to enable or disable the transformer driver
• Read as a status flag telling whether the transformer driver is in fault mode or not and specifically what fault it is
• Left floating to enable the transformer driver by default

Internally the pin is tied high through a 100-kΩ pullup resistor from VREG. This pullup resistor activates only after the VREG pin is high. If the UCC25800-Q1 transformer driver enters the fault mode, the DIS/FLT pin is pulled low internally via a 750-µA current source. When the pin is low, switching is inhibited.

The DIS/FLT internal pulldown current source is activated during the power-up sequence once the VCC voltage exceeds the UVLO rising threshold. After the VREG voltage has risen above the VREG_OK threshold, the pulldown current source is released and the DIS/FLT pin rises (unless it is externally pulled down). When the DIS/FLT pin voltage exceeds the EN_TH threshold, the transformer driver is enabled. When DIS/FLT pin falls below the DIS_TH the transformer driver is disabled. When the transformer driver is disabled its power consumption is reduced to IVCC_DIS.

If there is concern about noise coupling to the DIS/FLT pin it can be pulled up with an external resistor to an external rail or to VREG. In order to read the pin as a status flag, the external resistor value must be high enough that the 750-µA current source can pull the pin below the threshold level of the device reading the pin. It is recommended that the value for an external pullup resistor to 5 V is 10 kΩ and the value for an external pullup resistor to 3.3 V is 4.7 kΩ in order for the pin to be read as a fault output.

Figure 8-16 External Pullup for 3.3-V Supply

Figure 8-17 External Pullup for 5-V Supply

If the DIS/FLT pin functionality is not required, it can be left floating or tied to VREG to allow the transformer driver to operate normally.