I shell try to explain in the simplest way possible(In steps):
Forget about the LDO for now,lets look at the resistor case only.
For the diode to be conducting(forward biassed or ON),
lets assume a minimum voltage of Von(about 0.5V) is needed.
Thus the voltage at the Anode should be greater than the voltage at the Cathode by Von to produce a forward ON current.
.
Va - Vk.>Von..
Looking at the diagram I have put a constantly variable power supply instead of the battery.
Can you answer the flowing questions:
1. What is the diode's current at 10V power supply voltage ,is it ON?
2. What is the diode's current at 1V power supply voltage ,is it ON?
3. What is the minimum power supply voltage for the diode to be ON(forward biased) and what is it's current at that point?
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