New-Tech Europe | March 2016 | Digital edition

integrated current delivered therefore follows the profile of the reference signal. Various techniques can be used to control the reference signal and therefore manipulate the power output. For smart-home use, power converters can integrate additional inputs that tune the reference signal based on the preferred dimming level. The power converters for LEDs may also be designed to handle different supply types. For example, a simple retrofit bulb that includes the LED ballast, dimming control and RF interface to a smart-home network, will generally work with a 50 Hz/60 Hz mains supply. However, the power converters for bulbs that are designed to go into low-voltage strips and buses will need to be able to handle DC supply inputs instead. Inrush current is an important consideration in LED lighting design. The power supply to the LED presents a capacitive load to the mains supply,

which leads to the potential for a high current to be drawn for a short period when the LED is switched on as the capacitors around the supply charge. Because this current can be much higher than the steady- state current, it can lead to overload problems when a large number of lamps attached to one mains circuit are activated. Limiting inrush current at startup provides a design that has greater compatibility with home electrical circuits. Power-factor correction (PFC) is a further consideration in terms of mains compatibility. The switched- mode techniques needed for efficient LED dimming should not be allowed to drive additional harmonics onto the electricity supply. The need for efficient power delivery and support for PFC has led to the introduction of parts such as the Fairchild FL7701. The Fairchild FL7701 has been designed to work with both AC and DC supplies, using the ripple

voltage transmitted to its low-voltage VCC pin from the high-voltage supply as a means of detection. The ripple on the VCC rail is used using a sigma-delta modulation scheme to provide a reference signal to the PWM switching circuitry. When a DC input is applied, the lack of ripple on the VCC rail results in a reference signal suitable for DC-mode power conversion without PFC. To support remote control, the FL7701 has an analog input pin that can support 0 V to 10 V dimming control by using a resistive divider. This signal is fed into the digital controller and used to determine the output of a digital-to-analog converter that provides the reference signal to the PWM control circuitry. A microcontroller can be used to generate the input signal to the LED ballast, potentially receiving its commands from an RF interface. The FL7701 employs a soft-start function to limit inrush current, applying a delay to the internal voltage reference signal used by the PWM circuitry so that it rises

Figure 1: Block diagram of the FL7701

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