LED driver characteristics analysis
Based on the voltage-to-current rate-of-change characteristics, LED drivers require a design that meets the requirements, so understanding their characteristics and selecting the appropriate drive circuit based on a particular application is critical. This special 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 light, LED needs a forward voltage to let the current flow. So the LED driver to provide positive bias for the LED in order to make it light. LED light level or brightness is usually proportional to the size 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 the constant current drive circuit is the ideal solution to control the current level at the correct level of driving the LED. In other words, the LED driver circuit is a power supply circuit that provides a constant current rather than a constant voltage. The LED driver circuit should contain at least one voltage sense circuit and a current switching circuit.
When the voltage detection circuit detects the different voltage levels of the power supply, it will send a signal to the current on circuit, and then the current switch circuit is automatically activated, the use of a predetermined current value to adjust the electrical settings of the LED to re-adjust As much as possible for the LED.
The linear regulator provides a simple way to generate a constant current by connecting a current sense resistor between the regulator output and the grounded node. The constant output voltage of the regulator generates a constant current through the feedback resistor. Power supply reference voltage and current sampling resistor determine the LED current. Linear regulators are typically used to drive low-power LEDs, such as backlighting devices such as PDAs. The typical current values for these LEDs are between 15 mA and 25 mA and Vf is between 3.0 V and 3.4 V. If the linear driver is used to power multiple LEDs, these LEDs should be connected in series to ensure that the current through all LEDs is the same, so that the amount of light emitted is approximately equal.
The advantage of a linear driver is that the cost and electromagnetic interference of the solution is low because the linear regulator requires only a few resistors around the driver IC and does not use the switching element. Since the linear driver needs to output a 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 supply voltage is low. The main limitation of the linear regulator is that the supply voltage is always higher than the LED voltage, so the linear voltage source can not increase the output voltage, but can only reduce the voltage to a certain extent. This inefficiency can cause heat problems.
For high current applications with a wide input range, such as the simple drive scheme mentioned above produces higher heat generation and lower efficiency. And a constant current output of the switch driver is the first choice for driving high-power LED. The driver is typically used to switch the series inductance and LED load or parallel capacitors and the supply voltage on the LED. The inductor or capacitor is used to store the power when the switch is turned on; then the switch is off to provide current to the LED. Unlike a linear driver, the switch driver can be configured to achieve buck, boost, or coexistence. It is therefore clear that the switch driver allows the LED to operate over a wide input voltage range. In addition to the current adjustment function with constant luminous flux, they can minimize power loss. There is no doubt that switching regulators are more efficient than linear regulators. However, switch drivers are more costly and require careful design for EMI problems than linear regulators. In order to drive the LED in the right way, you need to find one of the most satisfying cost performance.
Many LED applications require dimming features, such as LED backlighting or architectural lighting dimming. By adjusting the brightness and contrast of the LED can achieve dimming function. Simply reducing the current of the device may be able to adjust the LED light. But let the LED below the rated current in the case of work will cause many adverse consequences, such as color problems.
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The way to replace the simple current adjustment is to integrate the pulse width modulation (PWM) controller in the LED driver. The PWM signal is not directly used 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 of the current LED chips use PWM to control the LED light. To ensure that people do not notice a 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 color difference when the LED emits light.
Other features of the LED driver
With the ability to emit more light than traditional lighting sources, high brightness LEDs occupy a place in many lighting applications. But these LEDs produce more heat than traditional LEDs. Therefore, the LED driver requires overheat protection to avoid damage to the heat that is issued during continuous operation.
An overheat protection circuit can be implemented with a thermistor to cut off the power supply when the temperature reaches its default value. In addition to overheating protection, there are other safety issues to consider, such as short circuit protection and open circuit protection.