PV has enough power to charge a smartphone, but every day when power reduces two (2) phones failed ( in succession?) with damage due to PV charger.
When PV source dims and voltage drops, smart phone's charger cuts out. THen PV voltage rises to an adequate threshold and a smartphone charger turns on again with a surge demand power and PV drops in voltage, and the cycle repeats while the phone surge power increases losses in SMPS battery charger that appears to be the likely cause of failure with constant oscillation on and off and just below cutoff underload but above cutoff with no load. We don't know the exact component that failed but it is dead.
Boost the storage capacitance with a battery as intermediate low impedance large mAh voltage sufficient to drive the smartphone charger port. This will reduce the fast damaging cycle rate to perhaps completely charge the phone until the next day or at the very least reduce the cycle frequency to many minutes or have the capacity to fully charge at least 1 smartphone.
Irrelevant to the problem ( aka XY problem)
- My previous answer ( for advanced readers)
An external voltage with some impedance is just a load current based on the difference voltage. As long as it does not exceed the LDO’s internal OCP and/or OTP it comes down to a load regulation error based on the device specs. \$Zout = \Delta V/ I\$ The lowest impedance that a PV can supply is Voc/Isc =Zmpt
THe open circuit voltage Voc and short circuit current Isc and Z max power transfer is the impedance you want to match with your load ΔV/ESR=Imax for maximum efficiency. Then Zmpt rises rapidly as the sun sets as Vmpt drops slowly. ( see PV MPT curves for a better understanding)
The same applies for a negative applied external voltage I= ΔV/Rs.
If the external voltage is greater the Zout becomes open circuit.
A “DC OK” logic level, can be made with a reference voltage and comparator within your spec’s operating range and some desirable hysteresis.
You may be able to predict and prevent these load effects if you can define the loads and power sources. If the source drops below LDO headroom cutoff, this can also be monitored with a R ratio and another comparator.
If you want to optimize your battery charger and PV power, you will want the output voltage between 70% to 82% of Voc (no load) and disconnect load when storage cap drops below Vcharge acceptable tolerances. This will create an undesired Relaxation Oscillator effect as the voltage rises with no load and drops with insufficient low impedance to start the phone charger SMPS which tend to have a surge start current and have higher conduction losses at low input voltage or as you expected drop the PV voltage too low and cook the phone charger from it’s lack of protection for UV lockout.
There is also a design for a Window comparator using dual comparators for UV/OK/OV that may be considered. where OK >UV threshold and < OV threshold.
The only solution for this frequent problem is to have a local regulated battery to store PV power between phone charge uses. Then previously stored energy can be transferred to the preferably smaller mWh battery in the phone and still have a DC OK signal enable the charger connection. Matching the voltages as close as possible to the load at 82%Voc during max PV solar input is one design goal to minimize series drop power loss.
There must be commercial solutions with a PV battery bank that already does this. Maybe, search , find buy or make one.
Otherwise give all pertinent specs for all inputs and outputs to start a better design with links to datasheets in question.