LED drivers require a compliant design, so understanding their characteristics and choosing the right driver for a particular application is critical. Such a dedicated drive circuit can provide the rated voltage and current for these LEDs , creating a good condition for their normal operation.
To achieve the purpose of illumination, the LED needs a forward voltage to allow current to flow. The LED driver therefore has to provide a forward bias to the LED to illuminate it. The level of illumination or brightness of an LED is usually proportional to the magnitude of the forward current. In addition, the current through the LED should not exceed the rated current specified by the device, otherwise it may cause permanent damage. Thus a constant current drive circuit is the ideal solution for controlling the current at the correct level of driving the LED. In other words, the LED drive circuit is a power conversion circuit that provides a constant current instead of a constant voltage. The LED driver circuit should contain at least one voltage detection circuit and one current switch circuit.
When the voltage detecting circuit detects different voltage levels of the power supply, it sends a signal to the current opening circuit, and then the current switching circuit is automatically activated, and the electrical setting of the LED is re-adjusted using a predetermined current value, thereby effectively lighting As many LEDs as possible.
Linear driver
Linear regulators provide a simple way to generate a constant current by connecting a current-sense resistor between the regulator output and the ground node. The constant output voltage of the regulator generates a constant current through the feedback resistor. The power reference voltage and current sampling resistor determine the LED current. Linear regulators are commonly used to drive low-power LEDs, such as backlights for portable devices such as PDAs. Typical current values ​​for these LEDs are between 15mA and 25mA, and Vf is between 3.0V and 3.4V. If a linear driver is used to power multiple LEDs, these LEDs should be connected in series to ensure that the current through all of the LEDs is the same, resulting in approximately equal amounts of illumination.
The advantage of a linear driver is that the cost of the solution and the electromagnetic interference are low because the linear regulator only needs to place several resistors around the driver IC and does not use switching components. Since the linear driver needs to output a very high voltage in order to provide LED current, the disadvantage of this scheme is that the efficiency is low, that is, the ratio of the LED voltage to the power supply voltage is low. The main limitation of linear regulators is that the supply voltage is always higher than the LED voltage, so the linear voltage source cannot increase the output voltage, but only reduces the voltage to a certain extent. This inefficiency can cause heat problems.
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