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In the cellular handset market, the inclusion of high pixel count imagers integrated into the handset is almost standard. As the resolution of these imagers increases, the need for brighter flash sources also increases. The use of Xenon flash bulbs has long been the main illumination choice in digital cameras. In the mobile handset board area for non-telephone related functions is sparse and the Xenon approach is often impractical due to solution size. Luckily for handset manufacturers, recent advancements in high power white light emitting diodes (LEDs) have been dramatic. Manufacturers of white LED flash diodes now have products that can deliver light outputs greater than 70 lumens, and can handle pulsed currents greater than or equal to 1A. Along with these advancements come new questions the handset designer must answer. How much PCB area can be used? What added flash related features are needed? How much power can the flash driver be allowed to use? How much illuminance is needed to take a decent picture? Answering these questions can greatly aide the designer in the selection of a flash LED driver.
Solution Size
One of the first questions a handset designer must ask is how much board area can be devoted to a camera flash? In the LED flash driver arena, two boost technologies are commonly used: Switched Capacitor Boost (Charge Pump) or Inductive boost. Of the two boost topologies, the switched capacitor implementation is typically smaller. Most switched capacitor parts require four ceramic capacitors and two external resistors. The typical capacitor value recommended for these applications is 4.7uF with a voltage rating of 10V (to help with DC bias losses). These capacitors can be found in 0603 cases sizes from a number of capacitor manufacturers. Total solution sizes at or around 25mm2 are fairly common when utilizing switched capacitor flash drivers. Some switched capacitor flash drivers come in a chip-scale package and can have a total solution size that is less than 15mm2. Switched capacitor solutions also have the advantage of being very thin. Depending on the flash driver package, the capacitors are often the tallest component in the solution.
Inductive flash drivers tend to have larger solution sizes compared to the switched-capacitor driver. A typical solution size for an inductive flash LED driver is closer to 35 to 40mm2 of board area. Inductive drivers typically require two capacitors (input and output) with average capacitor values of 10 μF and are available in 0805 case sizes. Inductive boosts require a rectification element that can handle the peak inductor current and output voltage. In a synchronous boost, a pass FET (typically a PFET) is usually integrated into the flash IC. This integration often causes the size of the IC package to increase over an asynchronous solution. In an asynchronous topology, the pass element takes the form of a schottky diode. The main area increase of the inductive boost compared to the switch capacitor boost is the inductor itself. Applications with flash currents approaching 1A, typically require a 2.2-4.7 μH inductor with a saturation current greater than 1.5A. These inductors rarely can be found in footprint smaller than 3mm x 3mm. The inductor also tends to be the tallest component in the solution. 1.2mm heights are not uncommon.
Features
Once the topology of the flash driver has been decided upon, the next question to ask pertains to the feature set required for the design. The first feature to address is the type of control interface. The basic flash driver usually has two control pins allowing for three to four modes of operation (ex. Shutdown, Indicator, Torch and Flash). If the designer does not need to change brightness levels dynamically, these simple control parts are usually sufficient. On the other hand, if a higher level of control is desired, many flash ICs have some sort of serial control interface. One of the most popular serial interfaces used is the Inter-Integrated Circuit interface (I2C). The I2C or I2C compatible interface not only controls the basic on/off functionality, but also allows the user to set the torch and flash levels dynamically. Other features such as the flash safety timer duration, inductor current limit level, and over-voltage protection level, if present, can be configured through the I2C interface. These serial interfaces are especially useful when the general purpose input/output (GPIO) lines of the microcontroller/microprocessor are limited.
Many LED flash drivers provide additional control pins to further aid the designer address system level concerns. Today's imagers typically have an external strobe/flash pin that alerts the system that a picture is being taken. This strobe/flash signal can be tied to many LED flash drivers directly via a flash enable pin. A direct connection between the imager and LED driver eliminates any delay that could occur between the two parts due to controller/software limitations.
One of the largest system level issues in the handset today is managing the amount of current drawn from the battery during a call/data transmission. The combination of the current drawn by the Tx/Rx power amplifier during a call/data transmission and the current drawn by the flash driver can exceed the maximum allowed battery current. Most handset designs allow operation to occur down to a loaded battery voltage of 3.2V before the handset goes into a reset state (VBATT-LOADED = VBATT_UNLOADED " (IBATT * RBATT_ESR)). To prevent a reset condition caused by the ESR voltage drop in the battery, some of the newer flash LED drivers have incorporated a transmission pin (Tx) to help minimize the current drawn by the LED driver during a call/data transmission. By asserting the Tx pin on these parts, the flash driver can force the diode current to a lower level in a very short time (less than 100 μsec.) preventing the handset from going into a reset state during a call.
Efficiency
The topic of efficiency is not new to the world of handset design. The higher the system efficiency, the longer the talk time the user can experience. Inductive boosts can yield high efficiencies over large input voltage and output current ranges. Switch Capacitor parts are limited to a few fixed quantized gains (2x, 1.5x, 1x pass mode) resulting in a lower average converter efficiency over the same input range. When evaluating LED flash drivers, the topic of efficiency takes on a slightly different meaning.
Converter Efficiency or LED Drive Efficiency?
When dealing with Flash LED drivers, certain efficiency losses must be considered to obtain the solution efficiency. In order to have a controlled flash or torch/movie light LED current, a boost converter must either utilize a current sink/source or a tightly controlled reference voltage along with a resistor to set the load current. While both solutions have advantages and disadvantages, it is important to remember that both methods introduce a power loss that does not get taken into account when calculating boost converter efficiency. The solution/LED efficiency takes this power loss into account.
Two different converters can have the exact same converter efficiency and have LED drive efficiencies that are 5 to 10% different. In the end, if the converter efficiencies are the same, the converter that introduces the smallest loss due to the current regulation element should be considered the more efficient converter.
LED Drive Efficiency or Luminous Efficacy?
Unfortunately, LED drive efficiency does not tell the whole performance story. For example, let's assume that we have two Flash LED drivers and two different flash LEDs. The first driver has a converter efficiency of 85% and the LED voltage is 4V at 1A of current. The second driver has a converter efficiency of 80% and a LED voltage of 3V at 1A. Both LEDs produce the same amount of light for a given current and both converters have a feedback voltage equal to 350mV. Using Equation#1, at a worst case input voltage or 3.2V, the first driver will draw 1.6A from the battery. Using this same equation, the second driver will only draw 1.3A from the battery. Despite having a higher efficiency, flash driver #1 draws 300mA more current to produce the same amount of light as flash driver #2. This example highlights the effects of the LEDs luminous efficacy. The LED in example #2 is 33% more efficient at producing light than the LED in example#1.
Converter efficiency, while important when flash LED drivers are running in a continuous movie/torch mode due to potentially long on-times, is not as important in the traditional sense during a flash condition due to the short duration. What is important is that higher efficiencies allow a higher amount of output power to be delivered to the flash LED for a given input power, allowing for a brighter flash. Picking a flash LED with a high luminous efficacy in conjunction with an efficient flash LED driver can help minimize the current drawn off of the battery during a flash event, allowing the handset designer more flexibility in the power management scheme for the rest of the system.
Optimizing Light Output
Optimizing the light output of the LED flash driver involves answering two key factors (three if we include cost): How much illumination is required for the handset imager, and how much power can be used to provide the illumination? For a given input power budget, there are three main ways to increase the brightness of a flash event to help the designer reach the illumination target. LED selection, LED current drive, and LED configuration all play a huge role in optimizing the light output of a given flash LED driver.
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