Flash LED drivers for mobile applications: overcoming the constraints of space and power

Technology News | January 29, 2012

By eeNews Europe

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.

Types of flash driver circuit

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 crippled 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 (see Figure 1). 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.

Fig. 1: efficiency graph for AS3685, a 700mA capacitive flash LED driver IC from austriamicrosystems compared to the inductive solution AS3648 with up to 2000mA.

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.

The efficiency of an inductive driver depends on the losses in the inductor, the value of the PMOS and NMOS resistance in the DC-DC converter, and the voltage drop of the current sink or source. Some flash LED drivers have a high-side current source, which allows for better cooling if the LED’s cathode can be connected directly to ground, but in some phone designs this is not possible. NMOS low-side current sinks have the advantage of lower resistance and smaller transistor size.

The other two options, xenon & supercap, are more suitable for camera centric smartphones. A xenon bulb provides a short and bright pulse of light, which results in very good picture quality and an unblurred image. In practice, however, a xenon bulb and the driver module is too large and too expensive for the thin smart phone form factor and bill-of-materials budget.

Improving the inductive LED driver

Picture quality provides a clear means of differentiation for smart phone manufacturers. This means that there is a race to deliver a flash LED driver that provides high light output from a small, power-efficient IC that offers features to ensure safe operation under all conditions. Two new inductive flash drivers from austriamicrosystems, the AS3647/8, show an approach that meets these testing requirements.

Fig. 2: typical flash LED system – for example with a Osram OSLUX LUW FQ6N

Figure 2 shows how a flash LED driver is implemented in a mobile environment. The Image Sensor Processor sends the strobe signal to trigger the flash driver. The TX mask signal is a protective signal: its function is to prevent the flash driver from drawing a high current at the same time as the power amplifier, which could cause the phone to reset – a highly undesirable event. So if a TX pulse occurs, the driver IC will reduce the flash current to avoid overloading the battery.

Fig. 3: block diagram of the AS3647 inductive 2A flash driver in the small WL-CSP13

The key to an inductive driver design is the current regulator, which should have a low compliance voltage. (A constant current is an essential requirement for power LED operation.) This current regulator can either be a high-side PMOS current source or a low-side NMOS current sink; the latter has the advantage of a lower compliance voltage than a PMOS current source. This in turn helps offer higher efficiency, as well as a smaller chip size and hence lower cost.

austriamicrosystems’ decision to use a low-side current sink was crucial: the topology’s lower power dissipation also means there is less waste heat to dissipate. This then allowed austriamicrosystems to use a smaller package while providing a higher light output than would have been possible with the high-side current source topology.

In smart phones, the battery output voltage gradually drops through the discharge cycle. When fully charged, the battery voltage is high enough to drive the flash LED(s) and the current sink without needing to be boosted. In this case, the power is taken through the DCDC by switchin on the PMOS constanly and therefore having an output voltage of the DCDC which is close to the battery voltage. The current sink will increase it’s resistance until the LED(s) draw only the specified current, with the excess current dissipated as heat.

After partial discharge, the battery voltage will drop and boost conversion will be required. The driver’s DC-DC converter is switched on, and regulates to a voltage which guarantees that the LED and the current sink can be driven with the specified current. The lower the battery voltage, the higher the current drawn from the battery will be.

The inductive driver topology, then, is capable of supplying the high currents required in powerful flash lights. What austriamicrosystems has demonstrated is that this technology can be packaged in a form that makes it suitable for use in space- and power-constrained applications.

Both the AS3647 and AS3648 are housed in a 2.2mm x 1.5mm Wafer Level Chip Scale package with 13 pins in a diamond array. This is a unique package for flash drivers, and makes it quick and easy to complete the PCB layout, because microvias are not needed. A further space saving is provided by the design of the IC’s DC-DC converter, which has an operating frequency of 4MHz. This produces low levels of input and output ripple, and thus enables austriamicrosystems to specify a small inductor.

Efficiency is enhanced by the very low resistance values of the NMOS and PMOS transistors in the DC-DC converter. A further advantage of implementing a high-power LED flash driver is that additional features become available. High-definition video image capture is now a selling point for smart phones, and with just two clicks users can upload their video to YouTube or share it with friends on Facebook. An efficient torch feature can enable the user to take video in the dark. A highly efficient DC-DC converter in the LED driver is essential to ensure that running the torch does not drain the battery excessively.

Conclusion

By using an inductive topology, the austriamicrosystems devices are able to deliver a high power output (AS3647: 1.6A single LED; AS3648: 2.0A dual LED) in a package small enough for use in the typical smart phone form factor. The resulting high image quality can be seen in Figure 4.