AND8138

During the conduction pulse, this current increases, and energy is stored in L1. When Q1 turns off, the current in L1 flows into capacitor C1 and through diode D1 to the output. Inductor L2 also contributes to the current in D1, discharging energy that was stored in it earlier. When Q1 turns on again, the cycle repeats. Some of the energy that was stored in C1 is delivered to L2, while diode D1 is reverse−biased. To understand the operation of the circuit it is helpful to realize that 1) The average voltage across an inductor is always zero (the winding is a dc short−circuit). Thus, the average voltage across capacitor C1 is simply the input voltage, since L1 is connected to the input and L2 is connected to ground, and 2) The current in an inductor cannot change instantaneously (the current in either inductor will be the same before and after Q1 turns on or off). Figure 2 shows the key waveforms of the circuit.

The AND8138 new 350 mA light−emitting diodes (LEDs) present some new challenges in the area of the power converters that drive them. They must be driven from a "current source" rather than a voltage source because, as with previous LEDs, their forward voltage varies from part to part, and with temperature. It makes sense that in the interest of stable operation the source should remain a constant current. A second consideration is that the original source of power may have a voltage that varies above and below the voltage of the LEDs that are to be driven. For example, the input voltage may be between 10 Vdc and 15 Vdc, while a series string of LEDs may have a voltage of nominally 12 Vdc, such as would occur with four 3−V LEDs. Given that no galvanic isolation is required between the input and output, what's needed is a nonisolated dc−dc converter that can handle an input that is below or above the output. The single−ended primary inductor converter (SEPIC) meets this requirement. AND8138 has an inductor input, providing smooth input current, requires only one switching transistor, and can operate over a wide range of input, both above and below the output voltage. Figure 1 shows the schematic diagram of an example SEPIC, designed for 5.0 V output and 10 V input.

The function of the AND8138 is as follows. A controller, U1, produces constant−frequency, pulse−width modulated drive to the switching transistor, Q1. When Q1 is on, current flows from the input through L1 and Q1 to ground.