Compared to previous incandescent lamps, LEDs have different power requirements. In addition, the optimal solutions for driving LEDs vary depending on the power level and overall system requirements.

In traditional lighting solutions, such as incandescent lighting, the load is resistive. The power consumption and corresponding brightness of the lamp are functions of the voltage applied to the lamp and the resistance of the lamp (according to Ohm's law).

The light intensity will increase or decrease with the change of input voltage. Taking car headlights as an example, when the engine starts, the voltage of the battery is low, so the headlights will dim; Once the engine starts running, the battery voltage will return and the headlights will return to standard brightness.

LED is fundamentally different from traditional incandescent lamps. The brightness of LED is controlled by adjusting the LED current, and the resistance of the LED load will change with the different loads applied. LEDs require a constant current, rather than maintaining a constant brightness through constant voltage and resistance. The methods for achieving the above effects vary depending on the power level of the LED.

For LEDs with very low power, a very simple circuit shown in Figure 1 is basically sufficient. Compared to R, the effective resistance of LED is very small, so the current flowing through LED is determined by V/R. Its drawbacks are as follows:

Brightness is a function of voltage. The change in V essentially changes the current, thereby altering the brightness of the LED. Returning to the example of car headlights in the early days - they are still very weak when the engine is first started.

This solution cannot take advantage of the efficiency advantages of LEDs. Because compared to the resistance of LED, R is very large, and most of the power is consumed on R.