Circuit design for driving high-brightness white LED dimming with constant current power supply

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Summary:

With the advent of the energy crisis, efficient lighting technology has received widespread attention. Light Emitting Ddiode (LED) is a device that converts electrical energy into light energy by using a semiconductor PN junction or similar structure. It has many advantages in high efficiency, low power consumption, low voltage driving and long service life. Get popular applications, such as various consumer electronics products - mobile phones, PDAs, LCD TV backlights and so on. High-brightness LEDs are an ideal alternative to traditional incandescent lamps because they have much higher lifetimes and efficiencies than the latter, and unlike compact fluorescent bulbs, these LEDs can operate at low temperatures. In order to improve the performance and application range of LED lighting circuits, this paper will introduce a cost-effective high-brightness white LED (HBLED) dimming method.

For HBLD, the on-voltage is as high as 3 to 5 V and the operating current is 0.15 to 3 A under high illumination conditions. The brightness of the LED is roughly proportional to the magnitude of the forward current flowing through the LED, so one of the key technologies for LED applications is to provide a power supply or drive circuit that is compatible with its characteristics. High brightness LEDs have two basic dimming methods. The first is a PWM (Pulse Width Modulation) dimming method that turns the LEDs on and off at different frequencies from 0% to 100% at certain frequencies greater than 200 Hz. During the on-time, the LED is fully current-operated, and no current flows through the LED during the turn-off period to ensure color consistency. The second method is to control the amount of current flowing through the LED string, which may cause the voltage of the LED string to drop and cause a slight chromatic aberration. However, if you observe an incandescent lamp that works with the dimmer turned on, you will see noticeable color changes.

High-brightness white light diodes are generally driven by a constant current source. Because as the LED gets hotter, its voltage drop will decrease, and if the LED string is powered by a constant voltage source, the power supply will continue to supply too much current, causing the output voltage to increase until the power supply reaches the current limit or LED. Invalid. Pulse width modulation mode is to switch the LED with higher frequency. The switching frequency is beyond the range that can be perceived by humans. It gives the illusion that the LED is always bright. Now the pulse width modulation method is widely used to adjust the brightness of the LED. In some applications, The dimming ratio can be up to 5 000:1. The commonly used LED drivers are Buck, Boost, and Buck-Boost. The LM3402 is a step-down regulator derived from a controllable current source. The input voltage range covers the entire automotive application. The built-in MOS transistor can drive up to 5 LEDs, which is cost-effective, and has a wide acceptance field and simple and practical lines. It is the leader among many LED driver ICs.

Typical application circuit diagram of LM3402

LED dimming circuit electrical schematic

See the uploading documentation for details (click to download)