For low voltage applications in the LED driver, the lift voltage converter is a good choice. The reason for this is that they can be used above and below the input voltage to drive the LED string (step-up and step-down), high efficiency (easy to reach more than 85%), discontinuous mode of operation can inhibit the change of input voltage (provide excellent line voltage regulation), peak current control mode allows the converter to adjust the LED current, Without complex compensation (simplified design), easy to achieve linear and PWM led brightness adjustment, switch transistor failure will not damage the LED and so on.
Fig. 2 shows the connection circuit between buck, step-up and voltage converter and led string. However, this method still has disadvantages: first, the peak current is controlled, because the use of discontinuous current mode of the pressure converter is a constant power converter. Therefore, any change of LED string voltage will cause the corresponding change of LED current; Another problem is that the LED Open-circuit state will generate a high voltage in the circuit to damage the converter, and additional circuitry will be required to convert the constant power converter to a constant current converter and to protect the converter without load.