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A DC-DC converter uses a voltage feedback to regulate the output.
Normally, a DC-DC converter is used in Constant Voltage mode. With negative side of the load connected to Ground and the positive side connected to the converter output.
Then a voltage divider connected across the load, connecting to the voltage sense of the chip so to maintain a constant output voltage.
For Constant Current mode, you do away with the voltage divider and use a single resistor for the feedback.
You do this by connecting one end to Ground, and the other end to the negative side of the load as well as the feedback of the chip.
Then the positive side of the load to the converter output.
The images below show the basic differences between Constant Voltage mode(left) and Constant Current mode(right).
LEDs fade over time. Under normal opperating conditions, (rated forward current at rated temp) this is normally 50,000-150,000 hours (about 6-17 years continuous)
When driven with too much current, it will just fade more rapidly until it goes short circuit.
A cheap keychain flashlight does not use a resistor to limit the current in its LED. It uses the internal resistance of its tiny battery cells.
Half-decent (Name-Brand)LEDs have a detailed datasheet that shows the range of voltage of a certain LED part number (mine are from 1.5V to 2.4V when the current is 20mA) and show a graph of the typical current when the voltage source is varied.
When the voltage is increased from 1.6V to 1.8V then the current increases from 2mA to 40mA. The temperature also affects the current. The LED gets hot which causes its current to increase which makes it hotter and it goes POP.
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