The datasheet for the LTC3226 states in the "Programming the Input Voltage Threshold for the Power-Fail Comparator" section on page 11:
For a smooth transition from normal to back-up mode, the PFI threshold should be set 50mV to 100mV above the programmed LDO output voltage, VOUT.
However, three out of the four design examples I have found do not do this.
|Example||Data Sheet page 1||Data Sheet page 16||Design Note 498||Demo Manual DC1735A|
|Delta||-187 mV||61 mV||-237 mV||-48 mV|
As you can see, the only example that has the PFI threshold greater than Vout is the example on page 16 of the datasheet.
What happens if the PFI threshold is not set between 50 mv to 100 mV above Vout?
How weird is a "non-smooth" transition from normal to backup mode?