Power-supply IC drives multiple LEDs

Bright LEDs are becoming prominent sources of light. They often have better efficiency and reliability than do conventional light sources. Although LEDs can operate from an energy source as simple as a battery and a resistor, driving a string of LEDs in constant-current mode can better match the luminance between the devices without needing to match LEDs for their forward-voltage drop. A switching supply also gives better efficiency than methods using a linear ballast resistor to limit the current.

The circuit in Figure 1 uses IC₁, the integrated offline Viper22A switching regulator in a constant-current configuration to drive two to eight 1W LEDs. The circuit operates by monitoring the voltage drop across the sense resistor, R₆, and uses this voltage as feedback to regulate the current through R₆ and the LEDs. An operational amplifier in the TSM103, IC₂, monitors the voltage drop across R_SENSE and compares it with the 0.175V reference, which resistor divider R₅-R₇ sets, and closes the loop to maintain a 0.175V drop across the sense resistor. The output of the TSM103 drives the optocoupler, IC₃, which transfers the feedback to the Viper22A on the primary side. Figures 2a , 2b, and 2c show the completed board and the top and bottom layouts.

The LED's drive current is I_OUT=V₅/R₆, where V₅ is the voltage at Pin 5 of IC₂. You could easily modify the circuit to drive 3 or 5W LEDs, as long as you respect the maximum power rating, by simply changing the sense resistor to set a higher current value. When you design a constant-current power supply, the design of the transformer and the operating limitations of the circuit directly determine the output-voltage compliance of the constant-current source. For a design that can drive two to eight LEDs in series, the voltage drop across the LEDs can vary from approximately 7V for two LEDs to 28V for eight LEDs. This output voltage reflects back across the transformer and in turn changes the V_DD voltage to the control circuit and the peak V_DS across the power MOSFET.

The designers of the transformer in this application considered three limiting factors: the allowable V_DD for the Viper22, which has a range of 9 to 38V for the undervoltage and overvoltage thresholds, respectively; the maximum wattage of 12W for the Viper22A; and the fact that the reflected voltage across the drain of the MOSFET, which takes account of the turns ratio [(N_P/N_S)V_OUT], added to the input voltage, must be less than 730V.

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