ti led lighting solutions
High-brightness LED lamp in the lighting more and more extensive use of the range. This article describes a simple "situational lighting" that lighting is only used several components. Three LED lights are used to supply the switching constant current regulators, and MSP430 microcontroller generated PWM signals to control three groups of brightness. Printed circuit board can be installed in glass table lamp in matte, can be used for indirect lighting LED spotlight.
Regardless of what the power LED lights, now usually a constant current power supply, MR16because the LED lights in lumens (lm) as the unit of the optical output power is proportional with the current.
Therefore, all manufacturers are designated LED light output (sometimes called optical efficiency), parameters such as angle and wavelength as a function of forward current IF, instead of the so-called forward voltage VF of the function. Thus, we also used the circuit for the constant current regulator.
High-brightness LED lights constant current
Most switching regulator on the market are designed for constant voltage source, rather than the constant current source. Only straightforward way to slightly modify the circuit can be a constant voltage regulator to constant current mode of operation. We did not use a common voltage divider to set output voltage, but rather to adjust the current sensing resistor voltage drop. Figure 1 briefly illustrates this circuit.
Figure 1 switching regulator can be set to the voltage source or current source.
LED lights dim
There are basically two ways to dim LED lights. The first is the easiest way is to use analog control, Led lightingdirect control of the current flowing through the LED lights to reduce the current to decrease the brightness. Unfortunately, this approach has two major drawbacks: First of all, LED lights and the current brightness is not completely positive correlation between the size; Secondly, the wavelength of light (the color) will change with the current changes, so that does not match the rating of the LED lights value; these two phenomena are the industry tried to avoid the problem.
More complex control is the use of constant current source, this power has been set for the operation of LED supply the rated current. Add a circuit, you can use the tag specified interval rate (mark-space ratio) rapidly switch LED lights, reducing the average distribution of light, and thus have lower brightness. Marker interval can be easily adjusted rate of LED lights to adjust the brightness, which is called pulse width modulation (Pulse Width Modulation, PWM).
The use of PWM dimming lights
In this paper, TPS62260, for example, that the way to construct a variety of PWM control. TPS62260 is an integrated switching components with the same step down converter to the general frequency of 2.25MHz frequency of operation. The circuit in Figure 2, we have a black mark the PWM signal will be directly connected to the EN (enable) pin feasible way. The switching regulator circuits are based on PWM signals and switches. Our results show that in this configuration, you can use the PWM frequency up to 100Hz. The advantages of this approach is simple: do not need to use other components, and in the switching regulator is disabled, the static leakage current is very low, so this is most energy efficient way. But the drawback is that LED lights enable pin for the high-level response to be delayed, because the switching regulator has a "soft start" feature: the device starts, the output current will gradually increase until it reaches the rated LED current date. In some applications, this increase of the phenomenon can cause problems, because the current is increased from the lowest level during normal operation, LED light emission wavelength also change. For example, in DLP projectors or LED-backlit LCD TV panels, the changes will not allow the foregoing occur, but in this demonstration, in general, the naked eye can not detect this phenomenon.
The second method (Figure 2 in red), is the PWM signal through a small signal diodes with the TPS62260 with the error amplifier input. In this circuit, applied to the control input of 600mV above the positive voltage, over-drive the error amplifier and the LED off.slide bearing This circuit is not used as start input, it is not soft-start function regulator influenced the start delay, LED which can quickly switch.
In Figure 2, a third possible way to blue. This method is the use of LED lights on the PWM signal controls the MOSFET. MOSFET can cause short circuit LED lights, LED lights more quickly switch. Is a constant current regulator mode of operation, the current will pass through LED lights or MOSFET. Disadvantage of this approach include increased energy efficiency MOSFET cost and the poor: there may be up to the power consumption of 180mW at 2Ω current sensing resistors. The advantage is the high switching frequency: experimental results, TPS62260 operation in this setting, PWM frequencies up to 50kH. |
