As shown in Figure 7-7, the open-drain DIAG pin can be monitored to confirm the device is operational, and the output data are valid. During power-up, the DIAG pin is actively held low until the high-side supply is in regulation and the modulator starts outputting data. The DIAG pin is actively pulled low if:

• The low-side does not receive data from the high-side (for example, because of a loss of power on the high-side). The modulator itself outputs a constant bitstream of logic 0's in this case, that is, the DOUT pin is permanently low.
• The high-side DC/DC output voltage (DCDC_OUT) or the high-side LDO output voltage (HLDO_OUT) drop below their respective undervoltage detection thresholds (brown-out). In this case, the low-side may still receive data from the high-side but the data may not be valid. However, the modulator itself outputs a constant bitstream of logic 0's in this case, meaning that the DOUT pin is permanently low.

Figure 7-7 DIAG and Output under Different Operating Conditions