The V_IN UVLO circuit makes sure that the device remains disabled before the input supply reaches the minimum operational voltage range. The V_IN UVLO circuit also makes sure that the device shuts down when the input supply collapses. Similarly, the V_BIAS UVLO circuit makes sure that the device stays disabled before the bias supply reaches the minimum operational voltage range. The V_BIAS UVLO circuit also makes sure that the device shuts down when the bias supply collapses.

Figure 8-2 depicts the UVLO circuit response to various input or bias voltage events. The diagram can be separated into the following parts:

• Region A: The device does not start until the input or bias voltage reaches the UVLO rising threshold
• Region B: Normal operation, regulating device
• Region C: Brownout event above the UVLO falling threshold (UVLO rising threshold – UVLO hystersis). The output may fall out of regulation but the device is still enabled.
• Region D: Normal operation, regulating device
• Region E: Brownout event below the UVLO falling threshold. The device is disabled in most cases and the output falls as a result of the load and active discharge circuit. The device is re-enabled when the UVLO rising threshold is reached and a normal start-up follows.
• Region F: Normal operation followed by the input or bias falling to the UVLO falling threshold
• Region G: The device is disabled when the input or bias voltages fall below the UVLO falling threshold to 0 V. The output falls as a result of the load and active discharge circuit.

Figure 8-2 Typical V_IN or V_BIAS UVLO Circuit Operation