2.4.3 Dropout-Safe Start-Up Sequence Feature

The start-up sequence management of the MCP16503 ensures predictable timing between subsequent steps even in the case that some power channels may operate in dropout conditions with moderate loading.

This is the case for Buck1 or LDOs, because their achievable output voltage range (up to 3.7V) overlaps the input supply range (2.7V-5.5V).

This operating condition in frequently encountered in battery-powered applications. For example, some loads designed for 3.3V nominal supply voltage may not be able to withstand the fully charged battery voltage (around 4.2V) and therefore they would require a front-end regulator, but they could still operate when the battery voltage has decreased low enough to push their front-end regulator into dropout.

For example, if the battery voltage was around 3.1V and the Buck1 output voltage was also set at 3.3V, it is desirable to still start Buck1 and proceed throughout the start-up sequence even if the POK (Power OK) threshold for Buck1 may not be reached, since Buck1 will still be delivering a voltage within the I/O operating voltage range. This would allow a better exploitation of the battery, because the cut-off voltage is no longer dictated by the onset of the dropout of the 3.3V regulator (Buck1) and by its POK threshold.

By means of a dedicated circuit that monitors the input-output differential during start-up of the potentially affected regulators, the MCP16503 can still ensure a proper start-up and report an out-of-regulation condition of the relevant voltage rail (i.e. POK = 0) after the start-up sequence has been completed. The MPU can then detect the anomaly and decide either to continue operation or shut down the system.

The relevant EC Table parameter that defines the acceptable level of input-output differential to continue through the start-up sequence in lack of the normal POK is the Start-up POK Bypass Threshold.