Based on voltage-current rate-of-change characteristics, LED drivers require a compliant design, so understanding their characteristics and selecting the appropriate driver circuit 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 to control 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 is passed through a feedback resistor to produce a constant current. 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 values ​​for these LEDs range from 15 mA to 25 mA and Vf is between 3.0 V 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.
Switch driver
For high current applications with a wide input range, simple driver schemes such as those mentioned above result in higher heat generation and lower efficiency. A switch driver with a constant current output is the first choice for driving high power LEDs. This driver is typically used to switch the inductance of the series and the LED load or parallel capacitor and the supply voltage on the LED. The inductor or capacitor is used to conserve power when the switch is turned on; then the switch is turned off to supply current to the LED. Unlike linear drivers, switch drivers can be configured to implement voltage buck, boost, or both. It is therefore clear that the switch driver allows the LED to operate over a wide range of input voltages. In addition to current regulation with a constant amount of illumination, they also minimize power loss. There is no doubt that switching regulators are more efficient than linear regulators. However, switching drivers are more expensive than linear regulators and require careful design for EMI issues. In order to drive the LEDs in an appropriate manner, it is necessary to find a most satisfactory performance-price ratio.
PWM dimming
Many LED applications require dimming, such as LED backlighting or architectural lighting dimming. The dimming function can be achieved by adjusting the brightness and contrast of the LEDs. Simply reducing the current in the device may allow adjustment of the LED illumination. But letting the LED work below the rated current can have many undesirable consequences, such as chromatic aberration.
A replacement for simple current regulation is to integrate a pulse width modulation (PWM) controller in the LED driver. The PWM signal is not used directly to control the LED, but rather controls a switch, such as a MOSFET, to provide the required current to the LED. The PWM controller typically operates at a fixed frequency and adjusts the pulse width to match the desired duty cycle. Most current LED chips use PWM to control LED illumination. To ensure that people do not notice significant flicker, the frequency of the PWM pulse must be greater than 100 Hz.
The main advantage of PWM control is that the dimming current through the PWM is more accurate, thus minimizing the chromatic aberration when the LED is illuminated.
Other features of the LED driver
High-brightness LEDs have gained a foot in many lighting applications due to their ability to emit more light than traditional lighting sources. But these LEDs generate more heat than traditional LEDs. Therefore, LED drivers require overheat protection to avoid damage from the heat that is emitted during continuous operation.
A thermistor can be used to implement an overheat protection circuit that cuts off the power to the LED when the temperature reaches a preset value. In addition to overheat protection, there are other safety issues to consider, such as short circuit protection and open circuit protection.
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