Off-Line LED Driver with True DC Output Current

LEDs appeared as a practical electronic component over fifty years ago and are still revolutionizing the general lighting industry by offering a light source with high luminous efficacy, longer life, and much more.

Among the challenges faced by LED application engineers is the design of the LED driver circuit. A wide variety of driver approaches exist, each offering its own set of advantages and disadvantages. The trade-offs involved can make the choice of driver topology and LED string arrangement complicated.

The HV9805 LED driver from Microchip is a flicker-free LED driver solution. As the market for LED lighting matures, low- and no-flicker capabilities are slowly being recognized as crucial LED lighting product options. Many products currently on the market produce significant flicker due to the driver design. Flicker, even when not immediately apparent to the eye, can lead to a variety of health related problems, such as eye-strain, fatigue, headache and some more severe health problems as well. Various institutions, like the IEEE, the Department of Energy and the California Energy Commission are studying the effects of modulated light and temporal lighting artifacts on human health, and have proposed recommendations and regulations to limit flicker content.

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The HV9805 offers a simple driver solution which provides true flicker free light yet maintains a small footprint and is low cost. True constant current is supplied to the LED string with a simple two stage topology. In the first stage, a PFC boost converter provides a DC voltage for operating the second stage. The second stage is a simple linear regulator operating in constant current mode that regulates the LED current to a constant value for flicker free operation. Both stages operate at high efficiency. The boost stage operates in the boundary conduction mode with quasi-resonant switching. The linear stage regulates the LED current while adjusting the bus voltage for minimum overhead voltage.

The HV9805 driver solution provides the advantage of flicker free operation with a very small footprint and at low cost. The boost inductor is a simple single-winding design. By nature of the topology, the LED string operates at high voltage and low current. The bus capacitor is relatively small as the energy required for operating the LED load is stored at high voltage instead of low voltage. The high voltage, low current layout of the LED load comes naturally to many lighting products where a distributed light source is required and is easy to cool due to the distributed nature of such an LED load. A secondary benefit of operating at low current is the higher luminous efficacy attained. For instance, the driver is a good fit for the newer Chip on Board (COB) LEDs where many LEDs are arranged in series to distribute power dissipation over a larger area.

The design of the application circuit can be arranged for dimming over a wide percentage of brightness. The boost stage can also be swapped for a SEPIC input stage to accommodate lower LED string voltages.

The HV9805’s ability to produce high power factor makes it a good fit for industrial applications where low demand factor is encouraged by the power company, and it can deliver up to 50W from a single driver, which translates to 3000 or more lumens .

Lastly, it is worth mentioning that the linear regulator is capable of protecting the LED load from overvoltage at the boost converter output. The load current is inherently limited by the regulator in case the boost converter output voltage suddenly rises as a result of an AC line voltage transient.

The Microchip HV9805 LED driver is an excellent choice for those applications that seek flicker free performance in an off-line setting. The driver provides a true DC current to the LEDs to eliminate any flicker during regular and transient operation, and is a simple and economic solution with a small footprint.

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