RGB LED context lighting
High-brightness LED lamps are used in a wider range of lighting applications. This article describes a simple "scenario lighting" that uses only a few components. All three LEDs use a switching regulator to supply constant current and regulate the brightness with three sets of PWM signals generated by the MSP430 microcontroller. The printed circuit board can be installed in a matte glass table lamp or an indirect-illuminated LED spotlight.
Regardless of the power of the LED lamp, a constant current is usually used as the power source because the light output power of the LED lamp in lumens (lm) is proportional to the current.
Therefore, all LED manufacturers specify parameters such as light output (sometimes referred to as light efficiency), viewing angle, and wavelength as a function of forward current IF rather than a function of the forward voltage VF. So, we also use a suitable constant current regulator in the circuit.
Constant current of high-brightness LED lamps
Most switching regulators on the market are designed as a constant voltage source rather than a constant current source. The constant voltage regulator can be changed to a constant current mode by simply modifying the circuit in an easy-to-understand manner. Instead of using a common voltage divider to set the output voltage, we use a current-sense resistor to regulate the voltage drop. Figure 1 outlines this circuit.
Figure 1 The switching regulator can be set to a voltage source or current source.
Dimming LED lighting
There are basically two ways to dim the dark LED lights. The simplest method of the first method is to use analog control to directly control the current flowing through the LED lamp and reduce the current to reduce the brightness. Unfortunately, there are two major drawbacks to this approach: First, the brightness and current of the LED are not completely proportional; second, the wavelength of the light (ie, the color) changes with the current, so that it does not match the rating of the LED. Value; these two phenomena are issues that the industry is trying to avoid.
The more complicated control method is to use a constant current source, which has been set to supply the rated operating current to the LED. With the addition of a new circuit, the LED light can be quickly switched on and off using the specified mark-space ratio, reducing the average emitted light and thus exhibiting lower brightness. Adjusting the marker interval makes it easy to adjust the brightness of the LEDs. This is called Pulse Width Modulation (PWM).
Use PWM to dim the lights
Take TPS62260 as an example to illustrate the various ways to build PWM control. The TPS62260 is a synchronous step-down converter with integrated switching components that operates at a general frequency of 2.25MHz. In the circuit of Figure 2, we have a black indication of the possible way to connect the PWM signal directly to the EN (enabled) pin. The circuit of the entire switching regulator is switched according to the PWM signal. Our experimental results show that in this setting, the PWM frequency can be used up to 100Hz. The advantage of this method is that it is simple: no other components are needed, and the leakage current is extremely low when the switching regulator is deactivated, so this is the most energy-efficient way. The disadvantage is that the high-level response of the LED to the enable pin is delayed because the switching regulator has a "soft-start" function: when the device is started, the output current gradually rises until the rated LED current is reached. In some applications, this rise can be problematic because the wavelength of the LED's illumination changes as the current rises from its lowest value to its normal operating level. For example, in the LED backlight of a DLP projector or an LCD TV panel, the aforementioned changes are not allowed, but in this demonstration, the phenomenon is generally not noticeable to the naked eye.
The second way (shown in red in Figure 2) combines the PWM signal with the small signal diode and the error amplifier input of the TPS62260. In this circuit, a positive voltage of more than 600mV applied to the control input overdrives the error amplifier and turns the LED off. Since this circuit does not use a startup input, it is not affected by the startup delay of the soft-start function of the regulator, and the LED can be switched quickly.
The third possible way in Figure 2 is shown in blue. This method uses a PWM signal to control the MOSFET on the LED. The MOSFET can cause a short circuit in the LED lamp, allowing the LED lamp to switch more quickly. The regulator operates in a constant current mode that passes through an LED or MOSFET. Disadvantages of this approach include increased MOSFET cost and poor energy efficiency: up to 180mW of power can be consumed in a 2Ω current sense resistor. The advantage is the high switching frequency: the experimental results show that the TPS62260 operates at this setting, the PWM frequency can be as high as 50kH.
Figure 2 Three ways to use the dimmer function
Heat dissipation
Operating temperature is an important parameter in the performance of high-power LED lamps, which can significantly affect the service life, forward voltage, output wavelength, and even the brightness of the lighting device. The higher the operating temperature of the LED lamp, the shorter the expected life. Therefore, the printed circuit board size we used for the experiment must be fixed on the back with a double-sided adhesive heat transfer material to fix the SK477100 heat sink (manufactured by Fischer Elektronik). To reduce the temperature from 61 °C (without heat sink) to 54 °C (with heat sink) when the LED is operating at full power. The heat sink also helps to dissipate heat into the various parts of the printed circuit board.
bright future
This printed circuit board can be used to perform more functions, for example, a slot on the board for the installation of Texas Instruments' Z430-RF2500 radio module. The eZ430-RF2500 kit contains two radio modules, one of which can be fitted with a rotary encoder (using the test pins of the microcontroller in the radio module) to establish a radio link to the LED board.
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