Smart phones are remarkable devices, but one function – the flash light used when taking pictures – has not made much progress since the first 'camera phones' emerged a few years ago. Recent development of power LED appears to offer a huge benefit to smart phone manufacturers: it is both very bright and very small.
But it has until now proved difficult to implement a LED flash light with the performance to match the very high image quality now available from smart phone camera modules. The main obstacle is the difficulty of implementing an effective flash driver circuit that meets the phone makers' requirements for size, cost and efficiency.
The camera inside the phone is, in fact, a camera module, which contains lenses, mechanical elements, the autofocus coil and the CMOS sensor itself. The Image Sensor Processor (ISP) may be placed on the camera module, or on the main PCB of the phone.
Today, there are four different types of flash driver on the market: capacitive LED drivers, inductive LED drivers, xenon flash drivers and supercapacitor LED drivers. Flash LEDs have a voltage requirement of between 4.0V and 3.3V, and so flash LED driver circuits must regulate the phone battery's voltage, which can fluctuate between 4.2V and 3.3V, depending on its state of charge.
Capacitive LED drivers are capable of providing approximately 700mA of flash current. They have been popular because they are easy to design in, small, and cheap.
They have two main drawbacks, however. Best suited to single-LED applications, they are severely constrained by the 700mA current limit, which does not produce sufficient light output for high-quality image capture. In addition, they offer only poor efficiency.
The feature that hampers the capacitive driver is that its charge pump can only multiply the battery voltage by a factor of 1.5 or 2. The unwanted excess voltage must be dissipated, which results both in inefficiency and heat. Many charge pump-based suppliers of flash LED drivers provide DFN or QFN packages for better thermal management, but this does not eliminate the fundamental problem.
Phone designers have therefore started to look to inductive LED drivers to address the shortcomings of capacitive drivers. Inductive drivers can produce current up to 2A, and can drive dual LEDs with higher efficiency. Dual LED designs produce higher light output, although of course they take up more space and cost more than single LED designs.
The operation of inductive drivers is inherently more efficient than that of capacitive drivers, which provides benefits both in terms of current consumption and thermal management. On the other hand, an inductive driver is larger, because it must accommodate a relatively bulky inductor.
"Flash LED drivers for mobile applications: overcoming the constraints of space and power" explores the LED driver tradeoffs; it originally appeared in EETimes-Europe.
About the author
Jan Enenkel is Reference Application Engineer with austriamicrosystems AG.
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