Lighting customers requiring high lumen output often prefer to use multiple strings of LEDs for two reasons: higher reliability, if one string opens up the fixture it is still producing light and lower output voltage which meets safety requirements. In addition, area lighting customers also want even brightness, high efficiency, and high reliability, especially because many LED lighting fixtures have very long lifetimes. There are several methods that designers can use to drive a series-parallel combination of LEDs, but each method has drawbacks. For example, a designer could use a switching regulator for each string. These designs, however, are complex and multiplying the design by several strings can be costly. Furthermore, switching regulators introduce EMI into the system, which can make the full fixture design much more difficult. For a simpler design, a lighting engineer could also use one ballast resistor per string. This solution is less costly, but is extremely inefficient and does not offer great current balancing, which means that fixture performance and lifetime could be inferior, as will be described later in this presentation. In summary, with existing series-parallel LED drive solutions, lighting designers must pick between performance, complexity and cost.