Offline LED lighting simplified

Introduction
With growing concerns about the environmental impact of traditional lighting methods and falling LED prices, high power LEDs are quickly becoming a popular lighting solution for many offline applications. To meet offline lighting requirements such as high power factor, high efficiency, isolation, and compatibility with TRIAC dimmers, previous LED drivers used many external discrete components, resulting in large and complex solutions. The LT®3799 integrates all the functions required for offline LED lighting, addressing complexity, space, and performance issues. The LT3799 controls an isolated flyback converter in critical conduction (boundary) mode for LED applications requiring 4W to over 100W of LED power. Its novel current sensing circuitry provides a well-regulated output current to the secondary side without the need for an optocoupler. Its unique energy-bleeding circuitry makes the LED driver compatible with TRIAC dimmers without the need for additional components. LED open and short circuit protection ensures long-term reliability.

No-Opto Operation
Figure 1 shows a complete LED driver solution. The LT3799 senses the output current from the primary switch current waveform. For a flyback converter operating in boundary mode, the output current equation is:

I _OUT = 0.5 • I _PK • N • (1 – D)

IPK is the peak switch current, _N is the primary-to-negative turns ratio, and D is the duty cycle. The IC regulates the output current by adjusting the peak switch current and duty cycle through a novel feedback control circuit. Unlike other primary-side sensing methods that require knowledge of input power and output voltage, this new circuit provides much better output current regulation because the accuracy is almost unaffected by transformer winding resistance, switch RDS _(ON) , output diode forward voltage drop, and LED cable voltage drop.

Figure 1: 20W offline LED driver using LT3799 and TRIAC dimmable

High Power Factor, Low Harmonics
The LT3799 achieves high power factor and meets IEC61000-3-2 Class C lighting harmonic requirements by causing the line current to follow the applied sine wave voltage. A power factor of unity is achieved if the current drawn is proportional to the input voltage. The LT3799 modulates the peak switch current with a voltage proportional to the input voltage, generated from the input voltage. This approach provides a power factor of 0.97 or better. A low bandwidth feedback loop maintains regulation of the output current without distorting the input current.

Compatible with TRIAC Dimmers
When a TRIAC dimmer is in the off state, it is not completely off. There is considerable leakage current flowing through its internal filter to the LED driver. This current charges the input capacitor of the LED driver, causing the LED to switch on and off randomly and flicker. The previous solution was to add a bleeder circuit, which included a large and expensive high voltage MOSFET. The LT3799 uses the transformer primary winding and the main switch as a bleeder circuit, eliminating the need for such a MOSFET or any other additional components. As shown in Figure 2, when the TRIAC is off, the MOSFET gate signal is high and the MOSFET is turned on, thereby bleeding off the leakage current and keeping the input voltage at 0V. Once the TRIAC is turned on, the MOSFET seamlessly changes back to a normal power supply device.

Figure 2: MOSFET gate signal and VIN

LED Open and Short Circuit Protection
The LED voltage is continuously monitored through the third winding of the transformer. When the main switch is off, the voltage of the third winding is proportional to the output voltage and the output diode conducts current. Once an overvoltage or LED open circuit occurs, the main switch is turned off and the capacitor on the CT pin begins to discharge. The circuit then enters hiccup mode as shown in Figure 3.

Figure 3: Output open circuit process

When an LED short occurs, the IC runs at the minimum frequency before the VIN _pin voltage drops below the UVLO threshold because the third winding cannot provide enough power to the IC. The IC then enters its startup sequencing mode as shown in Figure 4.

Figure 4: Output short circuit process

CTRL Pins and Analog Dimming
The output of the LT3799 can be adjusted through multiple CTRL pins. For example, the output current can follow a DC control voltage applied to any CTRL pin to achieve analog dimming. In addition, over-temperature protection and line undervoltage protection functions can also be easily implemented using these CTRL pins.

Conclusion
The LT3799 is a complete offline LED driver solution that provides standard TRIAC dimming, active PFC and well-regulated LED current without the need for an optocoupler. This high performance and feature-rich IC greatly simplifies and reduces the size of the offline LED driver solution.