Circuit diagram of boost converter with direct control of LED current

To control the current of the white LED , the current of the white LED must be detected. The simplest and most convenient way to detect the current of the white LED is to use a current sensing resistor in series with the white LED. In this way, the total output voltage required by the converter is white light. The sum of the LED forward voltage and the voltage drop of the current sensing resistor. In order to simplify the circuit and reduce costs, it is best to design the current-sense resistor so that the voltage drop across the resistor is the same as the feedback voltage of the selected converter. The voltage drop of the current-sense resistor should be as small as possible, so as to minimize power consumption. Too high a voltage will only increase power consumption. The sum of the forward voltage of the white LED and the voltage drop of the current sensing resistor is usually higher than the mobile phone battery voltage, so a boost converter must be used to provide power. Assuming that the feedback voltage of the converter is 500mV, the total output voltage must be between 4.1 and 4.3V. The boost converter with direct control of white LED current is shown in the figure.