A linear regulator provides a simple way to generate a constant current by connecting a current sampling resistor between the voltage regulator output and the grounding node. The constant output voltage of the regulator generates a constant current through a feedback resistor. The power supply reference voltage and current sampling resistor determine the current of the LED. Linear regulators are typically used to drive low-power LEDs, such as backlighting for devices such as PDAs. The typical current value of these LEDs is between Ma and Ma, and the VF is between 3.0 V to 3.4V.
If the linear drive is used to power multiple LEDs, the LEDs should be concatenated to ensure that the current is the same through all LEDs, thus making the luminous volume roughly equal. The advantage of the linear drive is that the cost of the scheme is lower and the EMI is low, because the linear regulator only needs to place a few resistors around the drive IC and will not use the switch element. Because the linear drive needs to output very high voltage to provide led current, the disadvantage of this scheme is low efficiency, that is, the ratio between the LED voltage and the power supply voltage is low. The main limitation of the linear regulator is that the power supply voltage is always higher than the LED voltage, so the linear voltage source can not increase the output voltage, but can only reduce the voltage to a certain degree. This inefficiency can cause fever problems.