Power supply design tips: low cost, high performance LED drive

As LED as a decline in the cost of production, LED utilization rate is higher and higher, in a variety of applications including handheld devices, automotive and architectural lighting. Its high reliability, The service life of more than 50000 hours) , high efficiency, 175 lumens per watt) And an almost instantaneous response makes it become a kind of attractive light source. However, drive the LED is a very challenging job.

controlled the brightness of the need to use a constant current to drive the LED, no matter how the input voltage constant current must remain constant. Usually, the LED will have dimmer requirements, for example, to monitor or building lighting brightness adjustment. Implementation of LED dimming has two methods: change the LED current or using pulse width modulation ( 脉宽调制) 。 Of the least efficient way is to change the current, because the light output is not a complete with linear change, current change and and in the current low to change its rating LED chromatography.

reminding of human perception of brightness exponential model is very important, completely dark need substantial changes on current. It has a great influence on circuit design, because 3% of all current adjustment error by 10% in circuit tolerance and load into the error of 30% or higher. Although the response speed is slow, but using PWM adjustable optical current waveform will be more accurate. In lighting and display applications, it is hoped that more than 100 Hz PWM, which will help the human feel the shine.

figure 1 MC33063 constituted a low-cost LED drive

figure 1 shows a drive is very simple and very low cost of a single LED step-down voltage regulator, the regulator imposed a rapid move light feature. An internal switch, it is based on the current limit comparator MC33063, oscillator, and internal interface. Usually used in voltage pins have shut down function. In this case, a more than 1. 25 v voltage will shut off the power, and opens a low voltage power supply. With the opening of the circuit, due to eliminate the voltage feedback, the controller to current limit/hysteresis model.

oscillator generates a will cause the power switch of the start pulse. This makes the input voltage is suitable for current sensing resistor, LED and inductance. When the current reaches about 350 ma current limit comparator began to turn off the power supply and induced current switch. Inductance reverse and more than the input current, voltage, thus make the fly-wheel diode conduction. Inductance and the LED current cycle, until the next switch cycle when open. The circuit is very suitable for a wide range of applications. For convenience and low cost of handheld portable equipment, white goods and automotive applications using a 40 v rated voltage and a 1. The switch of 5 a current regulator is very useful. Although the implementation of the hysteresis control and open function, there is probably a challenging but basic topology can be applied to a wider range of applications.

we build and test the circuit in figure 1. Figure 2 shows the shutdown command and thus lead to the LED current waveform. The LED can be realized easily in a 500 Hertz PWM dimming. Current waveform of the rise time and fall time is less than 100 uSec. If the LED can tolerate higher ripple current, the inductance can be a smaller value and the rise time and fall time can be shortened. However, 500 Hz PWM is suitable for most applications.

green = output current blue = closed ( 脉宽调制) Signal

500 uSec/div 10 uSec/div

figure 2 provides quick hysteresis current control PWM response

all in all, as long as it's not specifically designed to drive the LED, such as MC33063 switching regulator can move light work well. The error amplifier can be used as a closed function to provide the LED PWM dimming function, the current limit comparator provides quick response and accurate current Settings, and its built-in power switch implements a small simple circuit.

edit: Steve