It would lead to a difference between the internal resistance and turn-on voltage of light emitting diodes (LEDs) by the conventional LED driving methods due to the differences in process or material, internal resistance on LEDs is easily affected by the process, temperature or the length/location of the circuit wiring connected to the resistance, which generates errors on the equivalent resistance during actual operation, and thereby generates errors on the output driving current. To drive the driving current or a plurality of driving currents with different loads via one or more connected current mirrors would also lead to errors.
Please refer to U.S. Patent Publication No. 2006/0082412, the prior art discloses a method of utilizing operational amplifiers to improve current matching between channels. A multi-channel current regulator includes two or more channels, and each channel acts as a current source or sink for each respective load. Each channel regulator regulates the load current so that the load current is proportional to an input voltage supplied to the channel. An operational amplifier is shared between channels. Each channel is selected in a rotating sequence for connection to the amplifier. Each channel is selected to initialize a two stage refresh cycle. During the first period, the output of the operational amplifier is charged until the output voltage matches the driving voltage of the selected channel. During the second period, the output of the operational amplifier is adjusted until the load current of the selected channel is proportional to voltage Vset. The above mentioned circuit utilizes a selector to switch different LEDs connected in series, and determines which LED is turned on for illumination.
However, the device or circuit disclosed by the prior art has a more complex circuit, requiring a larger amount of operational amplifier elements and a higher cost on production.